This is a quickie, but hopefully it'll help someone out there.
I had to setup my laptop so two people could use it to talk on Skype: one audio input (the call), two outputs.
First, I installed Soundflower.
Second, I opened up "Audio MIDI Setup" (/Applications/Utilities/Audio MIDI Setup.app) and created an Aggregate Device (Audio -> Open Aggregate Device Editor). I checked all the output devices, and included the Soundflower device:
Third, I set the system's input and output to Soundflower:
Fourth, I opened up AU Lab (/Developer/Applications/Audio/AU Lab.app) and created a new… thing… with two outputs and one input. After hitting OK the right number of times and getting the main window up, I hit Window -> Show Studio, then selected the Aggregate device on the left. Finally, I checked both the "1" and the "2" on the left side of the "input" to send its output to both output 1 and 2. Some tinkering with the input and output streams (in the Studio) was also necessary.
Finally, in Skype, I set the audio input to the built in microphone (which was fine for me), and everything was peachy!