Gnomes don't have any particular religion; they don't believe in a higher power. Instead, they place their faith in themselves, their friends and their inventions -- a logical choice. This is why we don't see gnome priests or paladins -- they concentrate on their inventions and technology, not higher beings that might grant them mysterious powers. It simply isn't logical to gnomes to follow an unknown element; they'd rather take the elements that are available, tangible elements that they can see and manipulate to the advantage of all around them. To gnomes, it's all about making the world a more organized and comfortable place to live via inventions and technology that they create.
But with the revelations in Wrath, the gnomes have suddenly been confronted with their own origins and the realization that they were, indeed, created by a higher power. In Cataclysm, gnomes will finally have priests added to the available list of classes they can play. While there's been no information released as to why the gnomes are suddenly interested in the Light, or how they'll incorporate the Light's powers into their own unique blend of magic and technology, I've thrown around a few theories of my own.
In World of Warcraft, gnomes are allowed to be both mages and warlocks. The pursuit of these particular schools of magic is generally hand in hand with the zest for invention, and magic is looked at as something to be used to modify existing technology for the better. So there are a couple of possibilities that could be followed here -- perhaps with the revelation that the gnomes have that higher power that created them, suddenly they've regained a new interest in spirituality and wish to look a little more closely at the Light. Or, and this one is a bit of a stretch mind you, the gnomes observed what the blood elves managed to do with M'uru in Burning Crusade -- literally harness a being of Light and siphon the energy off of it -- and determined that perhaps they could do something similar without harming the creature involved in the process.
There are other theories out there, but until we receive an explanation, we'll never know for certain why the gnomes suddenly decided to follow the path of the Light, other than conjecture based on what little we know of the gnomish race. However, while gnomish presence in the world has been scattered all over, largely neutral and seemingly without focus, there is one thing above all else that the gnomes as a whole have been focused on, and it's something we'll see near Cataclysm: retaking their beloved capital city for good.
See, while the gnomes don't really have much in the way of politics, there are still a few gnomes that seek power. Remember Sicco Thermaplugg, the High Tinker's advisor? He was one of these rare gnomes, the ones who sought rank and power over technological advance. Sicco wanted to be high tinker and worked towards this goal relentlessly his whole life. He wanted to see Gnomeregan turned into a great kingdom, with himself as king -- the first true king in over 400 years. And if that meant relocating the dwarves, well ... so be it. However the gnomes, being a highly intelligent race, saw through his charming facade and decided he was too ambitious for the seat of high tinker. They chose Mekkatorque, his closest friend, instead.
Sicco went mad that day. All that he'd devoted his life to had been taken away from him in a single moment via a vote that he had no control over, despite his best efforts to sway people to his side. He wanted revenge, and he worked just as carefully on obtaining it as he did on trying to become high tinker. Gaining the trust and favor among the intellectually elite, he managed to get himself into the position of Mekkatorque's advisor. When the troggs began their invasion of Gnomeregan during the Third War, it provided him the perfect opportunity to ruin Mekkatorque's reputation and gain the rank of high tinker in the process. In fact, while it's unverified, there are rumors that suggest that Sicco not only knew of the trogg invasion before it began, but that he engineered the whole thing himself in order to set Mekkatorque up for failure.
It was Sicco's idea to leave the Alliance in the dark during the Third War as far as the trogg invasion went, and the rest of the gnomes agreeably went along with the plan. After all, in the priority of things, a world invasion by a dark power that sought to consume Azeroth rated much higher on the list than troggs that were only invading a single city. It was also Sicco's idea to flood Gnomeregan with radiation. While Mekkatorque was against the idea as first, it seemed like the best option -- and if a few gnomes died in the process, Sicco argued, it would be a worth sacrifice for saving the entire race as a whole.
However, something happened that caused Sicco to be locked inside of Gnomeregan as the radiation worked its way through the city. Along with many other gnomes, the radiation caused him to mutate into a leper gnome, driving him even further down the path of total insanity. After the damage had been done, Gnomeregan had a new leader, the self proclaimed "King" Sicco Thermaplugg, who ruled over the remaining leper gnomes and continued fighting back the troggs, as well as working on ways to conquer the rest of the world.
The story of Thermaplugg is an odd one in light of gnomish origins -- why would a creature descended from a robot crave power and global domination? Part of it can likely be attributed to the Curse of Flesh, which made all gnomes the creatures that they are today. Perhaps the madness and the craving for power is simply some sort of faulty programming that was passed down for generations until Sicco was born; perhaps Sicco heard the call of some sort of old god. Regardless, Thermaplugg betrayed his people in the worst possible fashion and gave the majority of them a death sentence, all in the name of a fancy title.
Sicco was naturally branded a traitor for his actions, and sentenced to death though that death has never been carried out. While many were sent into the city, upon their return and analysis of their claim it was discovered that the "Sicco" that was defeated was likely nothing more than a cleverly engineered fake of Thermaplugg, and the real Thermaplugg still dwelt somewhere within Gnomeregan's depths.
Oddly, it's not just Thermaplugg, the leper gnomes and the troggs that are running the show in Gnomeregan. Down in the depths near Sicco's quarters, there are groups of dwarves -- Dark Iron dwarves. Why are these dwarves, commonly found in Blackrock Mountain and the surrounding areas, allying with Thermaplugg? It seems unlikely that a faction of dwarves that serve Ragnaros would have any use for Gnomeregan or the leper gnomes within. For now, the alliance remains a mystery, though a look into dwarven history and politics will hopefully offer some theories and ideas behind the mysterious alliance.
With the death of the Lich King and the Alliance's focus no longer solely dedicated to Northrend and the Frozen Throne, Mekkatorque has been able to finalize, at last, a planned assault on Gnormeregan in an attempt to reclaim the city and finally bring Thermaplugg to justice. No longer quiet or withdrawn, the gnomes are quite open about discussing this with the Alliance and asking them for their help, regardless of race. After all, the silence between the gnomes and the others in the Third War proved disastrous for the gnomish race as a whole. But what will happen after Gnomeregan is retaken? What role will the gnomes play in the Alliance?
There is no "oh wait" moment for the gnomes, because the answer is the same as always -- they'll continue to help their friends when needed and fight their foes when they have to. With the introduction of the Bilgewater Cartel of goblins to the Horde, there might be a few skirmishes here and there, but gnomes by and large aren't going to be a major player in any political battles. They've got far too many other interesting ideas and prospects to follow. With the retaking of Gnomeregan and the events in Cataclysm, we may see their priorities shift to helping their allies with the sheer amount of devastation Deathwing will surely create.
The largest role the gnomes will play in the Alliance is an ally -- a strong, intelligent ally that will step forward to help anyone they deem as friend. Given the current situation of the other Alliance races, they may be asked to step up sooner than they think -- but that's a story for another week.