I had an initial post that listed some options (read first?). 

The focus of these lists is to consolidate systems capable of currently running Frontier (kernel.scripting.com) which is Mac or Windows OS dependent at this moment.

I have some additional links to small personal computers:

• Dynamism.com’s Notebooks
• PC version of the Mac Mini via Padova Technologies
• Evan Fledman on CES: Aurora Multimedia & Asour Technology [etmag]
• ExtremeTech: Cappuccino PC & Logisysus
• Apple Mini
• Engadget (Taiwanese PC-maker King Young’s Sumicom)

I also have found some links to three MP3 players that have software developer toolkits:

• Creative Labs (via Adam Curry)
• Archos MP3 & RockBox
• Neuros (developers)

And finally Jon Udell recently had some thoughts on this topic also that I think are worth studying.