BuddyPress.org

I’ve run in to this with importing a userbase to a Buddypress install. WordPress likes login names to be lowercase, with no spaces, and no non-alphanumerics (except, I think for dashes (‘-‘).

Users can, of course, make their display name anything they like.

The follow-on problem though, is with @username notifications in BP1.2+, which work on the login name and not the display name, so if a user has the login username johnsmith and the display name John, only @johnsmith will work.

(I’ve also just recently encountered a problem in that usernames with dashes cause @notifications to fail. That is, if a username is johnsmith, @johnsmith works, but if his username is john-smith @john-smith fails.)