A man has recently reported using his Wii's Mii history to catch his cheating wife (his original letter to the GoNintendo message boards is here). He discovered that his spouse's Mii had been spending nights Wii Bowling with another man's Mii.
I'm not sure how authentic this story is, and if it is, of course I feel badly for the guy, but, it's a funny outcome of a too-cute technology.
If you're so inclined, you can make your own Mii here (there are quite a few of these sites out there). Just be careful if you decide to use it for late night rendezvous with a secret digital lover.