Username conventions

I recently upgraded an old 1.1 installation to 1.5.3. In the process, I discovered that the conventions for valid usernames has changed. In my 1.1 site I used the login "super_user" - in 1.5.3, I wasn't able to log in. Searching the forums, I stumbled across a post that indicated that underscores are not valid in usernames. To fix this, I visited my database through phpmyadmin, and changed the username to "superuser"

I believe the restriction has been that usernames can be alphanumeric-only for quite some time (maybe since 1.2?)