iRig MIDI is a small device that connects to your device's dock connector and gives you 3 MIDI ports, in, out, and thru. It lets you directly connect MIDI keyboards, synthesizers, drum machines, drum pads, pedal boards and more using MIDI cables (it includes 2 cables).
iRig MIDI also comes with 2 free apps that you can use on your iOS device with your midi gear: SampleTank which is a sample-based sound module app and iRig MIDI Recorder which is a dumbed-down sequencer that records and plays back any MIDI sequence.
iRig MIDI also has a micro USB port (and includes a cable) that connects to any standard USB power supply source so that the iOS device can stay fully powered during long sessions.
At the end of the day, this thing could be really useful for musicians, as long as there are apps out there that support it and don't suck. Here's a list of the current apps that are compatible with iRig. The list, which includes successful apps like GarageBand, seems pretty good so far and I'm sure it will grow.
iRig MIDI costs $69.99.
The one problem is the cables are not standard MIDI cables, you need to use these weird cables (2 are included).