Guild Wars 2, you may very well see it as a long-running prelude to a full expansion. ArenaNet's lead content designer Mike Zadorojny recently went on record puncturing that particular balloon, saying that if the team manages these updates correctly, it will "probably never do an expansion and everything will be going into this Living World strategy." This stands in direct contrast to previous speculation and forecasts that the game would receive one on some sort of timetable.
According to Zadorojny, the end goal is to provide the same sort of content as in an expansion, simply at a faster pace with a more episodic nature. He likens the experience to television, providing constant motivation to tune in next week for something new and exciting. While he declined to offer hard numbers on the game's performance, he did say that the game is doing quite well and has no plans to sell powerful items in the gem store, with the current business model supporting the game without any need for an expansion.
[Update]: Game director Colin Johanson has posted a clarification on the GW2 forums.
Just to clarify a bit, as Mike said there are numerous teams beyond our Living World teams, and some of them are working on much longer term projects which we'll go into details on much further down the road.
It's entirely possible some of the types of content which you might traditionally find in expansions would be released through an expansion in the future for Gw2, and it's possible we'd try something different when it comes to integrating those type of releases.
We have no final plans one way or another about expansions at this time, and certainly haven't ruled them out, it's something we'll discuss more in the future.
Edited to add: Our primary focus right now is on making the core Gw2 experience as strong and compelling as possible, we'll release a blog post later this month detailing more specifically our plans for Gw2 in the second half of 2013.