I recently upgraded my Media Center PC from XP MCE to the Windows 7 RC to make sure there wouldn't be any problems. Well, in addition to updating the software, I figured I'd implement my plan to use a share on my NAS (mapped to a drive letter for my convenience) to house my library as that made synchronization/backups/etc. easier. Especially since I've added a couple other MCE computers around the house...
Anyway, despite adding the drive letter as a part of my library, the Media Center software won't add the MP3s to the library. Windows Media Player will play the music (so I know its accessible, etc.) but it also won't add it to the library. (I kinda figured they shared a library, but it was worth checking)
I've seen from ye ole Google that others are having this problem, and it was a problem in Vista's Media Center software as well - but I couldn't find a solution that didn't involve synchronizing a local directory with the NAS...
FWIW, I'm running the 64-bit edition. Somehow I don't think that matters though.