UID_MIN in /etc/login.defs
After upgrading an old Ubuntu laptop to 11.10, the full name of my user account in Unity’s “indicator session” applet was: [Invalid UTF-8]
This turns out to be Ubuntu bug #837246, and the reason was that I had set my UID to a number below 1000 in order to hide it from the login list of users. Thanks to the comments in this bug report I found out about the existence of the UID_MIN parameter in /etc/login.defs. Setting that to a number lower than my UID makes my username visible at the login screen, but it fixes the issue I mentioned above.