I believe this because there's been so much of "Dumbledore will save us" that it's dramatically necessary for him not to be there so the focus can be on Harry and crew.
I'm betting that Dumbledore will be dead or incapasitated rather than a traitor. I think it's a little more elegant for his problems to start in book 6 than for a rock to drop on his head one chapter before the big fight.
There's a small chance that it'll be a fair fight even if Dumbledore is there. Voldemort could turn out the be the front man for something/someone more powerful than he is, but the books don't feel that much like the Lensmen series to me.
The other possibility is that some other unrelated menace could show up to keep Dumbledore busy--this feels a little more likely to me since Rowling is very inventive and keeps adding stuff. It's not restrained and orderly art, but it's definitely part of what I like about the books.
Anyone else have theories based on what's dramatically needed rather than what can be shoehorned into the story so far?
My impression from a small sample of other fans is that most folks believe that Dumbledore knows what he's doing, and all his decisions are based on complete knowledge and large plans. This is probably the best premise for fan fiction, but I think it's more plausible that he's scrambling as fast as he can. He's running a school with random students and what seems like a very limited and weird teacher pool. He's also facing a complex attack by a fairly capable and very violent villain. Is there textual evidence either way about how complete Dumbledore's knowledge is?
Also, as I reread _HP & the Order of the Phoenix_, I'm thinking about how hard it might be to get wizard society to work at all. Magically capable individuals have a lot of power, and (I suspect) pretty good chances of using it anonymously.