It's only fun with the dance pads The Wii Dance Pads are wired AFAIK. Maybe a N800/N810 in USB host mode and a modified BlueMaemo that then connects via Bluetooth to the N900 which runs Stepmania and outputs to the TV could work? But it depends on the latency that this "Bluetooth in between" introduces.

Also, you would need both a N8x0 and a N900 to get things going. Given that you can have Stepmania on a CD (and use it on a PC) and you have to take the dance mat with you anyway means that there's no real reason to have it on the tablet (you could place the Windows binary of Stepmania + some songs in your MyDocs folder and hook up the dance mat + N900 to a Windows PC, though).