As we test USB devices, we will list them here. In general, to have any chance of working the device must be "OS X class compliant" and 16-bit. Let us know if you have any information about devices that are not listed here.
To use a USB audio device, you need to buy an Apple Camera Connection kit (US $29.99). Note that this accessory does not support powering iPad, and that iPad will not supply power to a USB audio device..
NOTE: As of iOS 4.2 (really 4.2.1) Apple no longer allows USB audio devices access to ANY power, so the list of devices that will work is now smaller.
Here is a link to a video showing the Sound Devices Mix Pre-D running our recorder.
These devices have been found to work the iPad. If using one, make sure to turn off the compensation filters in AudioTools (Settings->General).
ART USB Dual Pre
Seems to work well, has fairly high latency.
This device has unbalanced line inputs that may be of some use.
M-Audio Mobile Pre USB
Will work powered off the iPad USB bus, unless you turn on phantom power. To use phantom, you have to power it with a powered USB hub.
TerraSonde AudioToolbox 3C Color / Sencore SP495
If you have one of these units, you can use it as a USB pre front end.
We have not been able to get these devices to work on iPad.
M-Audio FastTrack series