Speaking to IGN, Guild Wars 2 director Colin Johanson stated that there are no expansions planned for the game at the current moment. While they haven't completely ruled any out, don't expect any soon.
Johanson states, "expansions are definitely something that we’ll potentially look at in the future. We don’t have a timetable on it. We’re open to it, but I think our major focus as a studio is making the living world concept as strong as possibly can for the players that we’ve got."
He also notes that it's "unlikely" that there will be a Guild Wars 3, because Guild Wars 2 will evolve, until it "becomes Guild Wars 2.5 or Guild Wars 3."
This is pretty great news in my mind. If you keep getting free content updates for this subscription free MMO, you never have to worry about paywalls into new areas. Since ArenaNet has been keen on supporting the game well into launch, it looks like you'll have plenty to do.