MIDI Touch for iPad – is -a bit like TouchOSC- made for creating your own controller stuff. not cheap for an app. $18 / 14,49€
Send MIDI over WiFi & MIDI compatible since iOS 4.2 – 4 use with cam connection kit, then..
or possibly just want a ready & done DAW Control? maybe this is for you – AC-7 Core for iPad
works with: Apple’s Logic
MOTU’s Digital Performer
Sony Vegas/Acid Pro
Presonus Studio One
..they say.. but it’s cheaper $5 / 4€