It'll actually be a while before I read it, but I wanted to get my speculation about the ending of the Harry Potter series down for the record. This was written in response to a thread on the topic over on Peter David's site.
This is all speculation based on no insider info, but spoiler warnings for those who don't want even that much.
I'd decided a while ago that the most logical ending, given that I don't think Rowling will kill off Harry Potter but will want to effectively kill off the idea of further books, is for Harry to have to effectively abandon magic in order to kill Voldemort and save his friends. So everyone is saved, but he can't see or be affected by anything magical, and nothing magical can see him (maybe even split off the "muggle" and magical worlds into separate realms).
Oddly enough, Keith Olbermann, of all people, had some speculation on his show a few weeks back which explained how this could be done. Harry's scar ties him to Voldemort, so Harry thinks he can only kill Voldemort by killing himself. Someone else (Olbermann suggested Snape, but after all the "Snape is evil, no he isn't" fake-outs, I want Snape to be evil, even if it does make Dumbledore look foolish) suggests the alternative of abandoning magic, and with the help of his now-reformed muggle family he does so.
Of course, even if I'm right, and I'm not at all confident that I am, I'm almost certain Rowling will leave herself an out in the final page (or something fans can embrace as an out whether it's meant to be or not). Or maybe allow Harry to take some of his friends with him to the muggle world.