For me, the solution was with a setting in the firewall Advanced Settings. The app "CIJScannerRegister" was set as "Allow incoming connections", but there was another item that was blocked: "canonijnetwork.bundle".
I set the "canonijnetwork.bundle" to "Allow incoming connections", and now all is well.
One thing... my iMac is quite new and does not have any legacy apps. Also, shortly after I bought it I upgraded to Lion. I don't know if that makes a difference for anyone else.