OBVIOUSLY Tobi isn't dead. He'll pop up again any chapter now, probably at a comically inappropriate moment in the middle of the coming Naruto group/Sasuke group face off, causing everyone to stop and stare.
If anyone remembers some of my earlier theories, I theorized that a) Tobi is close to unkillable and b) he hasn't let the other members of Akatsuki know the full extent of this (ninja always keep some of their secrets to themselves even from their allies, except Naruto because he's a doofus). So I don't think he would have had to make too much effort to survive even an explosion that big (the C4 on the other hand would have disintegrated him, which would have been too much). OK, that old theory is completely shot full of holes now (part of it was based on the idea that the AL is the 4th, which has been finally disproven) but I think the basics still stand. Remember, Sasuke cut Tobi down in one blow, but he stood up again without so much as an "ouch" (chapter 357).
The more obvious reason why Tobi isn't dead is because, as has been mentioned, he's an interesting character who as yet hasn't had a turn to show off any skills. Writing him out so quickly would be plain stupid, even by Kishimoto's (current) standards!