Last night I was able to do just this durring a recording session.
We needed to get a bumper so that both of us could hear it.
I've got a Peavy PV6 mixer, Griffin iMic, and Tascam 122L USB device.
The call in to Skype was getting routed to my Mixing board via the Tascam device and into my mixer's channel 5/6. I had the pan set to 100% Right. The audio out to Skype (and, thus, the caller) was going out from the mixer via the Left channel only into the griffin iMic from the Mixer's RCA line out jack. My Main out L&R from the Mixer were both going into the Tascam, and being recorded by Podcast Station.
To play my bumpers, I had the default audio device set to the computer's default, and had a connector line going from the computer's headphones jack to the Line-in 3/4 channel on the mixing board, with a 100% pan to the Right. My Mic (MXL 990) was on Channel 2, also with a pan 100% to the right.
As far as I know, this is the only way I've ever been able to share audio for interviews over Skype.
So to diagram:
Caller --> Skype --> out via Tascam headphone jack --> Mixer 1/4" input (Pan 100% Right) --> Main Out (Right) --> Tascam --> Podcast Station ONLY
iTunes --> Built-in sound card --> Computer Headphones or audio out port --> 1/4" Line In Jack on Mixing Board (Pan 100% Left) --> Main 1/4" Out (Left) --> Tascam --> Podcast Station
My Mic --> Mixer XLR Channel (Pan 100% Left) --> Main 1/4" out (Left) --> Tascam --> Podcast Station
ALSO out from Mixer: --> Line Out (RCA - Left Only) --> Griffin iMic --> Skype --> Caller