In iTunes 5.0.0, if you deselect the Radio option in iTunes preferences on a Mac or Windows computer, you will no longer see Shared Music libraries (from other computers on your local network) that are normally visible in the iTunes Source list. The Shared Music icon will remain in the Source list, but you won't see any shared libraries listed.
If you've opted to share your music library on your local network, it will still be visible to others, as it's not affected by whether you've selected the Radio option or not.
To resolve this, download and install iTunes 5.0.1 or later.