I think that it would be very difficult to implement different endings, if not impossible. Let's think it pratical. We remember the days in SC1 (and BW) where the campaings had to be followed in a certain order, simply because, the entire story was built in that order. If we played it different, we would miss the story. That method of multiple campaings - one story, was that gave us a continuation over the firsts games. I mean, if we play SC2 with diffent endings, it would'nt have a REAL ending after all (imagine if Kerrigan would have lost the last battle in BW). Creating "non-linear" endings, or alternate ones probably won't be Blizzard's matter, it would be player's.
If we are thinking on having multiple endings, on the way that SC story was built, it's a major task to create multiple campaings (like: Zerg - Toss - Terran | Toss - Zerg - Terran) and those with multiple endings. Maybe them, we could have different endings.
Althought the game itself might be going to the counterwise direction. It might have different endings (possible if SC2 will be the last game and have a real finale). I'm saying that because of the very amplification on the pre-game play we already saw in Raynor's campaing.

Sorry for the bad english, i'm not american.
My first post here.

Bye bye.