Blizzard is one of those companies that has a very firm policy on releasing new products. Namely, they release them “when it's done”, rather than setting a release date many months in advance. However, they have broken form and revealed when they are planning to release the latest WoW expansion.
According to a recent conference call with Blizzard co-founder and CEO Mike Morhaime, “we have said that it will be coming out this year,." That nails it down to sometime in the next five months, however, he goes on to say that "I can tell you it's not coming in the July to September quarter."
So it looks as though we can expect to see Wrath of the Lich King on store shelves in Q4 this year. If I were a betting man, I'd say it's more likely that they would get it out in October or November for the holiday season.