IK Multimedia iRig Keys I/O MIDI 25 - MIDI Keyboard, Portable Piano Keyboard, for Mac, iPhone and iPad, 25 Keys, 8 Pads, Music Production Station, Software and Applications : Amazon.co.uk: Musical Instruments
Finale Blog: Using your iPad as a MIDI Controller - Finale
The best wireless & portable Bluetooth MIDI keyboards for iPhone, iPad & Mac - 9to5Mac