I think he really is dead... He's in the portrait now, ya know? The Headmasters show up in the portraits when they die... But we never saw him take the Draught of Living Death. Harry saw Snape cast the Killing Curse on him. Only one other person has ever survived that, and for some reason I just can't picture someone surviving the Killing Curse through a Potion. If that was possible, wouldn't it be a possible solution to keep people alive who need to be alive? The DEs are here, quick, drink the Draught!

I think it was planned out though, merely because Snape wasn't as evil of a bastard as he should have been if he was really in the dark side.

*cuddles* Yeah, Dumbledore's death was really upsetting... However, it might just give Harry the extra little boost he needs to move on and battle Voldemort.