I use generic DDR pads designed for the PS2 through a PS2 converter, which I play stepmania with on my mame cabinet. It works perfectly.
I also use those PS2>USB converters for my guitar hero guitars to play Frets On Fire.
I'm running a 2+ghz P4 in my mame cabinet and I still can't run DDR fast enough in MAME, but Stepmania works great.