Steal comments that answer the question and post them as an answer. If it happens enough either the misguided commentators will be annoyed enough at the "lost" rep to give outright answers next time, or no one will notice / care and you'll help the community and... oh yeah, get some rep.
BTW, I stole this answer from Jeff Atwood's comment on sysadmin1138's post, so you should upvote this to annoy Jeff.
As Nixphoe points out in the comments below, sometimes you have to lead the author along to get more information out of him. In those cases, comments should be avoided like the plague and any information discovered in comments should be placed in either the original question or in an answer.
If you post an answer based on limited information, wait to see if more info is given and then edit the answer accordingly. It's also nice to modify the original question to include all information that is garnered in the process.
Throughout all of it, encourage the author to update their own question and provide more information. Eventually people will get the hang of it. The Q/A nature of the site is defeated if half of the pertinent information is in comments, and the accepted answer is only truly acceptable after reading two dozen comments strung out over the question and three different answers.
Always under all circumstances strive to keep all infromation in either the Q or the A.