I have just accidentally upvoted a comment rather than flagging it. I can't see a way to undo this (like you can do for a question) and once you've upvoted a comment you can't flag it.
I suggest it would be handy to be able to reverse a comment upvote and for upvoted comments to remain flaggable.
status-declined
in that? Funnily enough, you need to wait 5 seconds to undo the vote... – Daniel Beck♦ Jun 6 '12 at 6:21