iPad + djay app + iDJ Live (MIDI controller)
It's surprising, just how much of a full turntable set-up I'm able to replicate with an iOS device running Algoriddim's "djay" app ($20 for iPad, and $0.99 last I checked for iPhone.)
On the hardware side, DJ manufacturer Numark have made "iDJ Live", a $75 MIDI-based controller designed for the djay app. For $75, it is a pretty damn nice piece of work. Usable, and yet it doesn't feel (nor look) terribly flimsy.
Being able to instantly jump to a cue point and scratch makes for some creative options not possible on vinyl, since you're cheating physics with the iDJ Live.
On the iPad 2, you can also time stretch without pitch bending in addition to the BPM detection feature; both can be quite useful.