There’s an update available for the Windows 7 Beta that fixes several issues with Windows Media Player and Windows Media Center for Windows 7, one of which has potential implications for data loss in MP3 files. If you have installed Windows 7 on a machine with MP3s on it, or that has access to shares with MP3s, you should install this update ASAP. You can find more information, and a link to the x86 and x64 versions of the update, here:

http://support.microsoft.com/kb/961367