There are a few older questions, which are classified as “unanswered” but to which a decent answer has actually be provided in one or multiple comments. Sometimes, a comment has even been posted to ask the person who provided the answer to write it as an answer. If, after some time, nothing of the sort happens, is it okay for another user to write a community-wiki answer summarizing the comments (or copy-pasting, with attribution, if only one comment contained the answer)?
I like this proposal. Apart from saving future users the effort of interpreting all the comments, the question will also appear in the search results list as answered, making it more likely that the answer will be seen. If you're looking for an answer you don't jump to questions that don't have one, do you?
It will also raise our percentage of questions answered, which is one of the performance indicators of our beta.
To echo what Isaac said, there a numerous times where a comment is immediately fleshed out into a full answer by someone else. To be fair, you can prod the person who made the comment to provide an answer, but if some time has passed, then don't feel guilty for posting it as a full answer. In this case, I don't think Community-wiki is necessary.
While I personally agree with the idea that whoever posted the comment should be prodded and given time to make it an answer before someone else posts it as a CW answer, it has come up in discussions on various SE metas that it's not against the rules and not exactly against the spirit of the sites for someone to immediately copy the comment into a (non-CW, even) answer, though the spirit of the sites dictates attributing the content to the original poster—the notion that it's perfectly valid to take someone else's idea/post, with credit, and do something with it is central to SE.
(reposting my comment as an answer by request)