Long logins OD/AD…

We are experiencing a very strange occurrence in one of our Mac labs. It seems that groups of our students get a "stuck" login window. The login button turns gray after they click it, and then it just sits there forever. All of these students share one thing in common, a "firstname.lastname" user name format. The problem does not seem to occur with our other users who are using a "firstname lastname" user name format. The environment is an AD/OD setup, with the macs authenticating against AD and using OD to get preferences etc. The server(s) logs don't say much, but I have not enabled directory service debugging yet. AD users on PC clients with the firstname.lastname user name format have no issue logging in. Are there any caveats for using a firstname.lastname format for the username on the OD end of things?