http://www.anandtech.com/tradeshows/showdoc.aspx?i=2868

A lot of people here prefer consoles for gaming but gaming hardware/software is usually created for PC first and is then used with consoles afterwards. So if our bran new 360 is to truely take advantage of having three cores the PC market has to make some innovations first. As dual and Quad core cpus start to become the norm in the PC realm, PC devs will start to learn how to use them and make middleware for multithreading.

I found and interesting article that talks about this and why having multiple cores is significant.

Epic are using a multi threaded render for Gears of War, but i think it was Microsoft developed and will be distributed in future editions of development kits/environments.

Source and UE3.0 are going to be the engines of choice this generation. We should see some superb games running on them and since 360' s CPU is general purpose only, developers like Valve creating multi threaded solutions will mean significant improvements in engine performance.