I dunno about itunes, but if your xbox is connected to your home network and your computer is on, you can listen to any music that is on your pc. If you use your xbox as an extender and can even use the windows media center feature. You can also use Tversity to stream stuff to your xbox. Music, Pictures, Videos. I personally would use Tversity, its very good, try it. Once again, your xbox has to be connected to the same network your PC is connected to.
Another way is via. Last FM. You download the extension on the computer, configure it with your iTunes library, and you can choose this as a free station on the xbox. Otherwise, put it all on a memory stick, or use windows media centre. You can set up with computer from the xbox, but I don't think it works with iTunes.
I think Tversity is better. You can listen to any music that is on your computer. By default, only shared folders are accessible by Tversity, but you can manually share other folders as well. So you don't have to use your itunes and you can play your video games while listening. Tversity also decodes and stuff, so you can watch all types of videos on your xbox too.