Wired magazine just choose 9 best iPad gears for musicians (link to original article).
1. iPad camera connection kit
With camera connection kit you can use midi hardware and USB sound cards with you iPad. There are a lot of iPad apps that support midi inputs including GarageBand.
And here you can find lists of compatible and non-compatible midi hardware.
2. iConnect midi
This device allows you to connect any midi device (it has 2 midi inputs, 2 midi outputs and 2 USB ports). It has powered USB ports so it can be used instead of powered USB hub for devices that require more power.
With iConnect midi you can merge two or more MIDI input ports to a single output port and send a MIDI input port to one or more output port - it is fully configurable.
3. Avid M-Audio Keystation Mini 32
This is the very compact, "small enough to fit in a backpack" midi keyboard that you can easily connect to your iPad (with Camera Connection Kit).