Before the iPhone and the iPad made multitouch screens commonplace, there was the Lemur, a multitouch controller for synths, sequencers, and other electronic music instruments. The device has been used by musicians for quite a while, and it combines a touchscreen controller unit running a version of Linux with all sorts of musical electronic wizardry. Now, the Lemur interface has arrived on the iPad as an app, and it will connect via Wi-Fi to up to eight different computers to run any musical sequencers you happen to have around.
I'm sorry to say that I'm not quite musical enough to use or buy an app like this, but I can definitely appreciate the power here. This is the full Lemur interface (used by professional musicians for years) complete with a full editor, custom scripting, custom-made controls, and even physics for those controls (so you can "throw" faders around or bounce dials and switches). The back end sounds really incredible, too. You can connect right out to CoreMIDI or any USB device with the iPad's camera connection dock, or use that aforementioned Wi-Fi connection.
Sounds slick. The app is US$49.99, and it actually works on both the iPhone and the iPad. Reviews so far are great, so this sounds like a really cool piece of tech for musicians all the way up to the professional level.