I have a site where a person signed up and today he complained that he wasn't able to get on.
Doing a quick user search I found out that he actually has 2 logins. Both the same from an email point of view, but one with all lowercase and the other with mixed upper/lower.
I haven't looked in depth at this problem yet, but it appears that the registration/login procs don't do a lower on comparison. I thought it used to.