Looks like NLog MIDI Synth is coming soon from these tweets:
NLog MIDI Synth”: Polyphony, velocity, pitch bend, mod wheel, sustain pedal & assignable MIDI controller to synth params. In AppStore soon!
Historic moment: The first synth app making your IOS device a real synth playable with MIDI hardware: “NLog MIDI Synth” submitted to Apple.
Another app with Line6 MIDI Mobilizer
http://static.evernote.com/noteit.js 
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Can't wait to get my hands on it and check out the new layout. They should make it Universal also.
I want to also mention that the Line 6 midi mobilizer doesn't use a lot of power. You can probably get 6 hours out that thing connected to the iPad.
Yeah, not sure if it is universal or not. Hopefully we won't have to wait to long to find out.