It's in the release notes ( http://download.microsoft.com/download/4/5/6/45605938-AC68-4FF7-A092-10AFF0F7E5C7/Windows%20Server%202008%20R2%20Beta%20Release%20Notes.htm#ID0E6G ) however make sure that you are aware of this:
When MP3 files are added (either manually or automatically) to either the Windows Media Player or the Windows Media Center library, or if the file metadata is edited with Windows Explorer, several seconds of audio data may be permanently removed from the start of the file
Eeek. So if you want to let Windows 7 anywhere near your MP3 collection, either make them read only like the release note suggests or make sure you have installed the patch (http://go.microsoft.com/fwlink/?LinkId=139391) manually or visited Windows Update to get it.