Several people have mentioned that I end up costing them a lot of money; from showing off apps here. In the spirit of sportsmanship:
Run away! Abandon all hope and wallets, ye who enter this article!
As much as we all love to tweak around with things on our iOS screens, for actually playing a keyboard... you want a keyboard! I can't emphasize enough how much more I play now that I set up the SynthStation 61. It is just so much more fun and natural. I tweak the knobs and sliders on the screen and play the notes with the keyboard. Even if you don't have any experience playing a keyboard, you'll get better just by having fun with one.
A couple of iPad specific, 37-key, keyboard reviews have just come out from Kelly's Music & Computers, an online gear retailer. I usually dislike retailer reviews, but I think Kelly does a good job of getting to the stuff that matters to iOS Musicians. I also like that these are 37-keys he's looking at, as that is my personal minimum zone.
First up is the sleek and cheap GarageKey Mini 37. I checked and Kelly actually has the best price on this one, beating Amazon by $10, at just $59 on his company site. If you have small fingers, or a small area you're playing in, then these mini-keys will suit you.
That seems like a nice entry-level option, but I dislike the location of the octave switches. If you want some bigger keys and a bunch of knobs he's got a nice battery-powered solution with the Miditech i2 Control 37. This is $148 on Kelly's site and I can't find it available anywhere else in the US. $250 in New Zealand!