If you're using the Internet Junkuster Proxy, you may be experiencing trouble connecting to MP3 streams in iTunes.
The problem is that Junkbuster rewrites the user-agent field in http request headers. This behavior can be changed by editing the Junbuster configuration file. Depending on your configuration this could be located at /etc/junkbuster/config or (if installed via fink) /sw/etc/junkbuster/junkbstr.ini.
To fix the problem, simply uncomment the line:
#user-agent .or add it if it isn't already there (note that there's a period on the line!).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020523210139773