I found this bit searching from Apple's discussion forums. If you have difficulties purchasing music from the iTunes Music Store, but are able to access the previews and internet radio, this discussion has a workaround for the problem. Basically, the information there shows how to redirect the HTTPS requests from iTunes (which seems to ignore the proxy settings) to the proper HTTPS proxy.
I've tried it on my corporate network and it works, and am able to buy music now.
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Mac OS X Hints
http://hints.macworld.com/article.php?story=20031113222110416