If I look at what I do that needs crunch power, there's chess, but that wouldn't sell enough - 99.999+% of the chess players in the world can't beat the current portable chess computers.
Real-time MPEG-4 video encoding would be good, but you'd need too many other things (video grabbing h/w etc)
A portable audio recorder that does real-time MP3 encoding would be possible - use it as a portable (dictation, meetings, bootlegs etc) and attach it to a PC via the USB.
Paul, anything in the cryptography line? Phone scramblers don't need that much crunch, and I doubt there's the market for something to break codes the hard way (although it'd pay well)