That wasn't the Doctor in The Impossible Astronaut though and the eighth Doctor did say that he might've been dead too long for the regeneration to take place. The sixth Doctor's behavior after the regeneration was chalked up the suddeness of the regeneration.
Yeah, but the Doctor explicitly says in The End of Time (much as I'd like to write that story out of the canon) that he can die without regenerating. Seems reasonable to me. If you silently walk up behind the Doctor and shoot him in the back of the head, he'd be dead as shit. Makes the drama and tension of the episode thicker if regeneration isn't just a crutch in any fatal situation.