I'm in absolute awe of what I've just put together. I heard a couple months back that you can use an iTouch, among other devices, as a midi controller. I picked up my iTouch awhile back as my other portable music player went through the dryer one too many times, and, aside from being an apple product, it's been a wicked investment - way more so now.
With the purchase of a $10 program on your iTouch called touchOSC, some free software (for windows), and a bit of fuckin' around, you can have yourself a wireless midi controller that you can hold in the palm of your hand. You can use it for anything from a beat pad to a keyboard. Absolutely wicked. And, on top of all that for Ableton, you can also interface your device with anything else which is midi compatible... like Virtual DJ! So pumped.
Some videos showing what you can do:
Using Virtual DJ
Check out this HOW-TO to get you going. And if you're running Windows Vista like me, check out this thread to install MIDI OX correctly.