I bought the iDevice cable and associated programs. I put in fresh batteries into my Tonewood Amp, after about 7 straight hours of playing with the midi interface, I was forced to replace the batteries. After another 3 hours I can see that the batteries are going to need to be replaced soon. Due to the addictiveness of this device along with the midi interface I think you need to add this feature ASAP! =).
We have been considering this and it could potentially be a future option. The idea is to avoid being tethered to the wall when using TWA, however!
If you're not using rechageable batteries, try them! Our favorite is the Panasonic Eneloop Pro 2550mAh. We're getting about 8 hours per charge per set.
Having the MIDI cord attached to my iPad negates the not being tethered idea. The rechargeable battery option is already being used. Was noting the unused USB port as an option for power/