I fixed mine - so I hope this helps some people's cases (I'm Windows 7 32 bit, itunes 10.6.3, apple tv) - anyway it ended up being a firewall issue
In windows 7 goto start and type "Firewall". You should see an option to run "Windows Firewall" - click that. Once in click "Advanced Setttings". Goto Inbound Rules. Make sure any Bonjour or Itunes rule is enabled (I had a couple disabled - which is strange since I hadn't touched them --- but I did upgrade my system recently) - reboot and things started working
I tried a couple of other things - like switch firewall on and off, reboot my router, etc but I think it's the firewall settings that did the trick.