Maybe I should have known, but I recently had my Apple TV just stop showing up in iTunes. After following all of the troubleshooting tips at Apple's Support site, it crossed my mind that my new Checkpoint VPN might be the problem.
After some searching, I found this page that describes turning off the Checkpoint VPN SecureClient Security Policies. In your Checkpoint VPN SecureClient menubar drop-down menu, select Tools » Disable Security Policy.
I guess the security policy put in place pretty much blocks all incoming traffic to your Mac, including many Bonjour requests such as those from the Apple TV.
Mac OS X Hints