I found an answer -- Airfoil 4.0.2. Costs $25 but does what i need.
I have been enjoying streaming Pandora to my AppleTV2 from my 2ndGen iPod Touch -- occasional hiccups, but overall not bad. Since this setup seems to rapidly sap battery strength on the Touch, I'd like to know if there's a way I can stream Pandora to my AppleTV2 from my iMac or MacBook Pro via AirPlay or standard WiFi. I've done a quick search and found nothing relevant. Is AirPlay exclusive to iOS devices?