The thread http://mathoverflow.tqft.net/discussion/973/where-is-the-hole-in-my-proof/ -- and related discussions linked to from that thread -- are sort of relevant to precedents and consensus-forming regarding these sorts of questions.
Basically there was a fair bit of agreement that asking for evaluations of preprints was off-base; published papers are fair game for MO queries.
Questions of the form "is my proof correct" may be considered okay if and only if the 'level' is right. For example, Qiaochu Yuan observed that often
the basic problem is this: if you already have a proof, you should already be capable of checking it yourself. If you can't readily check it yourself because you don't understand concept C well enough, you should ask a question about concept C instead of a question about your proof.
A common example of the wrong sort of question is where someone lazily links to his pdf file and asks people to give a thumbs-up or down.
On the other hand, Ben Webster observed that
a question of the form "Here are two facts, it seems to me like they contradict each other. Why am I wrong?" is basically acceptable (maybe because I've been annoyed by so many of them). That's completely different from asking people to vet a preprint (which I think is mostly what people have complained about in the past).
That's good because it typically gets right down to the mathematical meat in an engaged and serious way.
