Software update 2.1 (build 888.2) provides a fix for the metadata glitch that we’re reading about in these forums – that is, when you upgrade from 1.3 to 2.1, your music collection displays incorrect album/artist/track info when playing on the Zune software. This update resolves that. Here’s how to install:

If you’re still running 1.3:
When you’re prompted for an update via the Zune software, accept it. This updated build – 2.1 build 888.2 - will not encounter the metadata glitch.

If you’ve already upgraded to 2.1
IMPORTANT NOTE: If you’re already running 2.1, you do not need to install build 888.2 – the software will tell you “you’re already up to date.”

This link will help you recover if you've encountered the metadata issue:  http://support.microsoft.com/?id=945319