I was thinking of buying some kind of "air" Bluetooth keyboard but then I discovered the Yamaha MD-BT01 MIDI-to-Bluetooth adapter which allows you to use ANY keyboard with 5-pin DIN MIDI ports wirelessly with your iPad. It sells for $50.

The choices of "air" keyboards isn't real good and I like the idea of being able to use different keyboards with my iPad so I was glad to see the MD-BT01 come along. I have one on order so I can't testify to how well it works yet but it's gotten good reviews.