Speaking to IncGamers at BlizzCon, Blizzard Co-Founder Mike Morhaime said that although Blizzard would like to be able to release one expansion for WoW a year, and that's still the aim, it's unlikely to happen.
As we all know, Blizzard will not release a game until it is ready, no matter what. They had no problem with missing a Christmas release for the Burning Crusade, and the same goes for their previous claim of releasing one expansion per year. So we shouldn't expect the next expansion until 2010 then?
"The next one? I think it is still unlikely that we're going to get it down to a calendar year. We're going to continue to try and decrease that, but really the content of the expansion is more important."
However, we can probably expect to find out what the next expansion will be at next year's BlizzCon.