In the interview from MMO Report, Chilton indicated they wanted to push expansions every year and a half, but they had to push the release of Cataclysm back several times to finish zones they never intended to change very much. Let's look at the track record (not counting Vanilla, since its themes for raids were all over the place):
Burning Crusade - 2.4 patch made solely as a filler patch because of development slag in WotLK (and because the "big boss" of the expansion was rushed)
Wrath - 3.3 added in December 09. 3.3.5a added another raid in June to tide people over until their then-internal target release of September (by June/July they knew they would have to push the date back into Black Friday, and later December)
My prediction:
Deathwing in a January/February patch. They will be targeting for an August/September release that will probably be pushed into early November. Filler patch? Hopefully, those will happen BEFORE the big end-expansion boss rather than AFTER, but history shows that will not be the case.