How about an optional ‘solved’ or ‘answered’ flag that the original poster (or moderator) can apply to a thread when they have a satisfactory answer to a question? Some forums put that flag into the title of the thread and/or highlight the most helpful response to-date directly after the question. This is especially helpful for seaching for only ‘answered’ or ‘open’ questions, and when threads increase in length over multiple pages.
We have the option with our current forum module to enable question & answer voting (similar to stack overflow). I agree that we need a means to mark questions as “resolved.” Do you think that all users should be able to vote on answers to a question, or just the person who asked? Also, would it be too acrimonious to have down voting?