One hint: a preview of Algoriddim's Djay for iPad. The app lets you cue up and play two songs from your iPad's music library at the same time. It provides a crossfader, EQs, BPM detection, beat matching, and other essential features for spinning sans vinyl, and with an optional attachment it even offers cue previews. Yes, you can use the touchscreen interface to scratch and yes, you will definitely look like a douchebag doing it (assuming iPad DJs become ubiquitous enough that you won't already look like a douchebag just being one).
A video preview after the jump:
Of course it's not going to sound anywhere near as good as vinyl, and of course it's not going to provide the kind of tactile interface that serious DJs demand, but for your average bar gig Djay should do nicely. It definitely beats dragging around crates of vinyl. I respect the "analog for life" crowd and all, but I also respect not having a chronic back injury.