In the US at least there isn't a rule banning the use of mirrors.
But FWIW, the few times I've raced with guys using them I haven't been impressed. There was a guy in a stage race this spring sporting a helmet mirror, he seemed to be compulsively glancing up every few seconds but since the mirror was mounted on the left of his helmet he all but ignored everything happening on his right. First he overlapped wheels a couple of times with his constant glancing up and back and secondly he probably had a good idea of what was behind him on the left but when it came time to move right he just came straight across without so much as a glance under his arms. He bounced off more than one rider by doing just that and seemed clueless when folks gave him flack for not looking before moving. After the first stage I did my best to be ahead of that guy as he was a menace.
That was the worst case I've seen in racing but have seen other guys, probably regular commuters who've gotten used to using mirrors in traffic use them really poorly in races. If you're going to ride and race in the peloton you've got to develop the skills to glance down and under your arms to know where you are, who's around you, who's making a move, where the safe holes are and you've got to be able to do this while riding up top or in the drops. A handlebar end plug style mirror might help in some situations, but you still can't ignore what's happening on your right side or what's happening outside the mirror's limited range of vision.
-Dave