Wow, that looks really nice even in its early stages!
Definatley be keeping an eye on this with it being WPF based, I have much respect for the other frontend's out there but most have great features but are let down by poor visual appearance - with WPF you have the opportunity to make some sexy visuals for us to drool at
I'm going to chuck a few ideas in here for things I feel make a good FE they may help you in some way ?!;
- Browsing Albums by cover - like WMP11's cover view (much easier than text to see on the move!)
- Single tape album loading - eg. Tap the album cover to play, tap and hold to get tracks seperatley
- 'Jump to' buttons for music - eg. Buttons on music view for 'ABC' 'DEF' etc.. single tap of 'ABC' jumps to albums starting 'A', double tap 'B' etc. - Nobody wants to search 500+ albums by an arrow button

- Telephone Signal / Battery / Status available to 'home' screen
- Customisable 'sections' eg. I would never use the Web browser so would like to turn that off completely so it didn't appear in the FE.
Look forward to seeing more of this FE develop!

hopefully it won't die premeturely like so many others before it
If you need any testing done under Vista or XP just shout!