I believe the Nuforce Udac-1 works well the the iPad. I received the following from Nuforce re. the new 24/96 uDAC-2 -
"uDAC-2 is not compatible with iPad, at least for now. There is a software bug and Apple is aware of it. They will be fixing it but we have no idea when it will be released. The iOS doesn't know how to recognize any USB DAC > 16bit, 48kHz."
Mike