I searched a bit and found references to using SoundFlower and other 3rd party tools to enable multiple audio interfaces on Mac OSX. There's a built in way to do the same thing outlined here:
http://www.apple.com/pro/techniques/aggregateaudio/
I have a mbpro with an Alesis Multimix8 firewire. The Multimix only has 2 outputs, so I created an aggregate device with the muitlmix + the built-in input/output. This allows me to use the cue features through the built-in headphone jack to route the click to my drummer's ears only. Awesome!