You may have already read in the news that Facebook announced that its users will be permitted to create personalized usernames/brands for their Facebook pages. It is a first-come/first-served basis starting 12:01 AM (Eastern time) Saturday, June 13, 2009.
Facebook has an on-line form for holders of registered trademarks to complete to prevent third parties from registering their trademarks as usernames.
More information can be found at http://www.facebook.com/help.php?page=896 and the on-line form can be accessed at http://www.facebook.com/help/contact.php?show_form=username_rights.
As part of its program to permit users to create usernames/brands, Facebook has also implemented a method for trademark owners to report trademark infringement and Facebook is reserving the right to remove or reclaim any name at any time.