Nick, realizing you've got an iPhone , and that the iPad/iPhone4 support midi, there are quite a few iPhone Midi Controller apps available for your iPhone too.

Here are just a few of the many available for download via iTunes:
http://www.macworld.com/appguide/article.html?article=135437
http://www.crimsontech.jp/eng/m16/index.html

to discover others, do a google search
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