Last year when we released Exchange 2010 SP1, we posted about the change expired password feature in Outlook Web App and how you can enable it by creating the ChangeExpiredPasswordEnabled registry entry. See So you want to change your expired passwords in OWA... for details.
Since then you've sent us many requests for supporting the use of User Principal Name (UPN) format in the same feature. Previously only domain\username format was supported when entering domain credentials.
We passed on that feedback to Exchange PMs and we're glad to let you know that the support for UPN credentials is now available in Exchange 2010 SP1 RU3-V3, which was released recently.
Note: At the time of this writing, Update Rollup 4 is the latest rollup available for Exchange 2010 SP1.