I asked a question some time ago: http://stackoverflow.com/questions/3849726/
The comment was the holy answer, and I'd like to accept it as good answer. However, I can only accept answers.
Why can't I accept a comment as a good answer? Thanks.
Just ask the commenter to add the comment as an answer. Most people will happily do that.
If after several days nothing has happened, add an answer of your own, referring to the comment, and accept it.
Leave a comment directed at the poster (use @username so they get notification) asking them to repost the comment as an answer.
The only danger with that is that someone else comes along, sees the comment and posts an answer themselves, but hopefully the original poster will see your comment first.