You do know that Stepmania can AutoGen steps for songs, right?
Annnnyways....
The whole iPod DDR idea is sorta...ick. Konami did officially release a Cell Phone Version of DDR (with real MP3s IIRC), and a few people like it, others call it crap.
But as (possibly) the biggest Bemani/Rhythm&Beat Game freak thats on this board, I can tell you that the best "portable" DDR/Rhythm&Beat machine would be a mid to hi-end laptop. You can get almost ANY peripheral for a computer, from dancepads, to Popn controllers, GuitarFreak controllers, Beatmania/IIDX controllers, Drum Controllers, etc. You can pretty much find a free simulator unit for PC as well, and be set and ready to go! (After you pick up song songs as well)