In general I like the idea of such app. But I would rather prefer little bit austere UI and more finger friendly controls. Attached is just a quick sketchup I made (links to full sized .png). I have even more ideas in the head already but didn't have time to incorporate them (namely the waveform area showing the whole song overview and zoomed beats together - something like in one of the djdecks user made skins).

One other thing, how would one realize the prelistening of the track? Would be the master routed to i.e. left channel and cue to the right? This would degrade the production to a mono master output :-( With the external soundcard connected via USB it would be easy (H-E-N required then) but the drivers would be the issue I guess.

Anyway, even the version without prelistening would be fun! Keep going!