A tool that can be used to identify accounts that aren't yours can be found at: http://tools.wikimedia.de/~interiot/cgi-bin/queries/SUL.
Very useful link, thanks! There are 23 User:Tango's on Wikimedia's servers. I think about 5 or 6 of them are me. I was contacted a couple of days ago by an en.wikiquotes admin about an account someone had created there with the same name as me - it wasn't me, so they blocked it. I'd better go through and try my standard passwords against some of those accounts and find out which are me...