The Mvisto Pocket DivX and ISO Player (
link) looks like a perfect mobile media player. Powered by Linux, the player supports HDTV-quality video and Dolby Digital/DTS-quality Audio sources. Its best feature is its size: merely measuring 132x80x17mm, Mvisto uses a built-in 40GB 2.5" HDD to store full-length movies and audio tracks (I assume you could always replace the latter with a 120GB 2.5" drive). Suggested retail price: ~250 Euros (around $325).
Click here to access the full review over at I4U.