Well you should know that Blizz has already put the Arthas ending spoiler in the middle of Dalaran. If a guild has completed ICC on your server, a big shiny statue of the heroes appears in the middle of Dalaran with a plaque on the front. Clicking the plaque automaticaly drops you into the end cut scene, ala Wrathgate event, which I assume is the cutscene that raiders who complete ICC get to see too.
It was kind of annoying that there was no warning about what I was going to be getting by clicking on the plaque, but I suspect that its part of Blizz not wanting their endgame content only being seen by 2% of the server. Especially as they made such a big deal of shoe-horning Arthas into just about every questline in Northrend, I guess they want everyone to know how it ends;
"Go kill 20 scourgepigs and bring me their snouts.. even though only 1 in every 5 have snouts... and watch out for Arthas, he'll probably jump out and tell you the scourgepig lord is actually his minion and its all part of his plan. And when you get back I'll give you a few coins and this shiny leather hat!"

@Grak

I was wondering what the hell that giant thing was.

@Grak I think there was more to the decision to put the fountain in than just their view on people seeing content. First of all, the people who won't be able to do the raid still won't see the content by clicking it as it just shows a cinematic.
Secondly, the Lich King/Arthas has been a huge part of Warcraft lore since before Wow; Warcraft III to be specific. Many players who joined the universe then feel that the Lich King/Arthas has been the major pivotal role of the entire experience. It's hard to argue against that fact. Yes, the Burning Legion set it in motion, but it was Arthas's relationship with Sylvanas that shaped her personality and the direction of an entire faction. In fact, around the same time, he helped to inspire a complete about face for another faction; the High/Blood elves. It was Arthas, under the direction of the Lich King who helped to shape several zones. And of course, his actions after Northrend which created the whole who-should-have-Lordaeron arguements that abound on the official forums.
Of course, the Lich King was the grand puppet master, but Arthas was center stage for it all. It's hard to find another character that had as much impact, so far. It makes complete sense for Blizzard to want everyone to know what happens with the Lich King/Arthas storyline, regardless of whether they raid or not.
However, I agree that a pop-up box warning you that they're going to spoil the storyline and asking whether or not you want to watch would be a fantastic addition.