Onoko has released Pad MIDI (App Store link), a free Wireless MIDI control app for iPhone/iPad.

Features:

• Supports native Mac OS Network MIDI (Core MIDI)
• 16 assignable / renameable MIDI pads
• Full 88 key piano
• Selectable MIDI channel and velocity

There’s a tutorial on setting up MIDI on an OS X network at Nix Fox-Gieg’s Tutorials.