We've managed to snag an early copy of WMP10 for Windows XP. There are various new features with WMP10 including the following:
The UI is slightly different from WMP9 and it looks a little cleaner. The build is listed as 9.00.00.3409 but the installer says Windows Media Player 10 so we're assuming it's at a really early stage. We're still playing with the build so if we notice anything new we'll update the post. For those interested WMP10 is part of the Windows Media Center Edition 2004 Update beta (MCE 2005) that has been shipped to Media Center testers this week. Screenshots below.
Update: We've had to remove the screenshots at the request of Microsoft.