Stream audio from Windows 7 to Ubuntu
Solution 1:
For streaming just music, Icecast is a good option.
There is not a way to access iTunes shares from Linux, other than running iTunes under Wine.
You could also look at Airfoil Speakers which would allow your Windows iTunes to treat the Ubuntu box as a set of remote speakers.
Solution 2:
Depending on the hardware, you may be able to just connect the linear audio output on the Windows machine using an appropriate male/male 3.5 mm cable to a microphone input on the Ubuntu machine, and enable/increase volume of the mic input in the sound applet
Solution 3:
With Ubuntu, you should be able to access itunes shares with daap module that Rhythmbox can use, as well as Amarok. I've done it with my colleagues in the office before.