Here is a list of some of the iPhone and iPod touch apps that feature both background audio and virtual MIDI capabilities, and can therefore be ‘played’ by the AUUG app. Note that the prices that we have noted here are subject to change:

app_list

Here is a more extensive list of apps that have virtual MIDI as a feature, and the number of apps that are compatible with the AUUG app is continually growing:

http://iosmusicandyou.com/ipad-iphone-music-app-resource-pages/ipad-iphone-music-resources-apps-with-virtual-midi/