I'm currently working on a PIC-powered serial extender design to allow multiple serial devices to connect to the empeg (GPS, OBD etc), and also 1-wire, a couple of ADCs. I eliminated USB as a possibility because I don't have any USB experience and the lack of USB master on the empeg. I'm now wondering if the 2 ideas could be merged...

As to USB uses, for starters;
1) Keyboard - for entering addresses for GPSapp, searching for tracks, shell console.
2) Gamepad - empeg games are rather limited by the buttons.
3) *Big* on my wishlist - the means to plug in a flash card (SD, MMC, MemoryStick etc) writer. This would allow a flash card to be used to log data from eg GPSapp or OBD, or allow the empeg to be used as an image store during long road trips.