If you are using Tiger there is a great new feature...
Aggregate Audio Devices
This lets you combine multiple audio devices into one core audio device. Open the "Audio MIDI Setup" application then choose Open Aggregate Device Editor from the Audio menu. You then choose which devices to combine.
The only issue is that they get generic numbers for input and output so it takes a bit of experimentation to figure out which number is which input or output on which device.
It is a core audio driver so works all your apps.
I've used this with Soundflower and my MOTU ultralight to wire sounds internally as well as use the physical in and outs. Works well.