I personally love the Line 6 Midi Mobilizer, but If you search the web for iphone midi support you will finde many people (e.g.on KVR board)
that want a solution like on the ipad.
You only need to connect the camera connection kit to it an can plug in an keyboard with generic USB midi support.
Unfortunately this doesn't work on the iphone.
It used to work accidently on an iphone 4 with IOS5 beta 6 but in the official IOS 5 release this feature is disabled.
see this video:
This solution would be perfect for iphone musicians who want to jam on the go.
Imagine you could connect a mini usb Midi keyboard like the Akai LPK 25 or the Kork Nanokey (and there are a lot of them) the iphone and play your Synths! These Mini Keyboards you can take everywhere.
Wouldn't USB MIDI Host it be the perfect feature for the next generation Midi Mobilizer something like an t?