In the however many years I've been playing World of Warcraft, there has always been one constant that has stayed popular opinion since the games launch: Gnomes. People hate gnomes, for whatever reason they use to justify it. I've heard every excuse in the book from "They're short" to "They're so cheery and annoying," but none of the reasons have any real meaning behind them -- it's just popular opinion. The other side of the equation are the people that love gnomes and think they're the best thing since sliced bread, and will not tolerate any disparaging remarks about this tiny and affable race. With Cataclysm's launch comes a new class for the gnomish race -- gnomes will now be able to roll the priest class and heal right along with their Alliance brethren that have been doing this for years.
By and large, gnome priests have already existed in some fashion -- Gnomeregan had the presence of a group of gnome "medics" that were healers. When you're dealing with a society that is constantly mid-invention that may or may not accidentally detonate, it's probably a good idea to have a medic or two on hand to patch people up. But these medics didn't really seem to follow the path of the Light, something that dwarves and humans have been using for years as a tool for healing. Gnomes, however, are very good at inventing -- so it may be that they simply found a way to utilize their creations to heal people, hence the medics.
The gnome priest of Cataclysm is an entirely different creature, however -- these gnomes appear to use the Light just like their dwarven friends. There are two ways you can look at the gnome priest in Cataclysm -- the easy way, and the more difficult (yet in my mind, more entertaining) way. Since gnome medics already existed in Gnomeregan, and even now players on live servers are working to retake their irradiated home, you could simply assume that the gnome priests are medics that were rescued. Or we can look at gnomes and their past -- and how recent discoveries may be altering how they view their future in a big way.