Sometimes I see comments mentioning edits to questions and I confirm that the poster's reputation is under 2000 before proceeding to make the edit (since I don't want to accidentally clobber his edits if possible). Also sometimes a user will ask a question about SO itself as a comment and in order to answer him properly I need to know what his reputation is so I can tell him what his options are.
I'm wondering if anyone else would see any use for having a tooltip for the usernames in comments. Mouse over a user, and it would pop up at least the reputation number of that user, if not his entire flair box.
It would save me a click every now and then. I might be the only one, though.
It would also be useful for Community Wiki questions and answers, as they don't list reputation like normal questions and answers do.