I don't totally trust Dumbledor either, but I don't think he is a death eater or anything and there's no way he could be Lord Voldemort. If he was Voldy, then how in the world would the two of them been able to fight each other that night in the Ministry of Magic (it'd be kinda hard if you asked me)?
I think he is definitely holding something(s) back. After all, he does feel like a parental figure towards Harry so there will always be that urge to protect Harry no matter how angery Harry gets.
Like Tuitus said, I think the biggest secret was the prophecy, but there has definitely got to be something. Besides, if there wasn't, that might make the next two books rather dull.
- That Old Black Magic