ReBirth for iPad
Mix dance tracks through virtual sequencers and other hardware... Wow!
Pick up a default track, and start tweaking the nobs and sliders. That's all it takes to mix your own dance track, mess with the bass, add high and low notes, and all the rest.
I'm sure once I figure out how to copy/paste sequences and tweak and entire song I'll post the results right here in the ol' iPad review/tech blog. In the mean time, if you like to mix your own tracks and make music on the go, it's hard to beat ReBirth.