There isn't a lot of documentation (or even a Help file on the Apple site) but a quick look doesn't indicate so.
There is support for an Electric guitar, via a dongle, that uses a number of software Amps and Stomp boxes. But there is no mention of a similar support for Midi keyboards. I haven't plugged mine in to try it out, but I don't think so.
I should have tried it first. Just got home, plugged in my M-Audio USB keyboard, and it plays just fine. Not a lot of volume coming out of the little speaker, of course, but it sounds great through earphones.
As noted above, you do need the Camera connection kit to give the USB something to plug into.
I have the Roland UM-1G Cakewalk device that I used with my white Macbook and my Roland TD4 drum kit, and the user Rich C. (in comments) says that one works as well... VERY excited to get my iPad at the end of the month!!
Depending on what generation of ipad you have you can either get the 32 pin usb to camera kit or the Lightning to USB camera kit the adapter they run for about 30 dollars at retail stores you could probably find one online for a cheaper coast but beware the Chinese adapters may not work with you device properly I would recommend getting a genuine Apple adapter http://http://www.ipaddisti.it/wp-content/uploads/2010/08/camerakit.jpg
the kit allows you to put a usb enabled device int to your iPads charging port
Question How to hook up midi keyboard?
Answer :Plug a usb b cable aka printer cable or a midi to USB adapter in to your keyboard or controller then plug the usb end in to the camera kit then plug the kit into your device and load GarageBand and your keyboard or controller then becomes a midi device allowing you to control GarageBand with your device
Sometimes when you plug a device in it will not work and your iPad will say on screen something like this device may not work with this iPad or device not compatible or this device is not certified and may not work reliably with this device the reason for these errors are some devices require drivers and the iPad can supply the driver for it is not a full on computer
Something's when you plug in a device it will say the device attached requires too much power to get around that error you can plug in a powered USB hub into the USB camera kit then pug your device in to to usb power hub then the iPad should have enought power to use the device
hope this helps enjoy and ps I'm sorry I saw this so late if I would of seen it in 2010 I would of posted this sooner