With any luck PHB II will finish up Arcane, Divine and Primal, leaving PHB III & IV open to all new power sources, probably two per book.
Late 2009 PHB III: Psionics & Shadow
Early 2010 PHB IV: Elemental & Ki
Or any combination thereof.
Then the rest of 2009 should see the Divine Power book, Draconomicon II: Metallic Dragons, the Eberron CS, PG and Adventure, the 2nd and 3rd Epic tier adventures and way too many minis and dungeon tiles.
I don't know... I mean, from a marketing standpoint, think about it.
If a given person doesn't like Psionics nor Shadow (as you pointed for PHB III) why would he buy the book? I think, and might as well be wrong, that they'll keep supporting "old" Power Sources so that there's stuff for everybody's tastes on every new book.