I have three Macs on the same local network, all running iTunes 11 (one running 11.0, the other two running 11.1). As of today, suddenly none of the computers' libraries appear on any of the others as a shared library.
On all three computers, library sharing is enabled in iTunes preferences, Shared Libraries are set to be visible (under General), and Shared Libraries are not set to be hidden under Parental Controls.
Nothing with my network setup has changed, and I ran Network Diagnostics and everything is normal. None of the computers have even been rebooted!