When allowing SQL traffic to flow across an ISA server its good to know what the ports it uses are for. SQL primarily uses two ports: 1433 (the default sql port) and 1434 (the sql browser service’s port). If you specify that you want to connect to a named instance then the traffic will first be sent over port 1434 to the sql browser service. Then the sql browser service will send back the port number that the named instance is on. Finally your application will try to talk over this port that was given to it by the sql browser service.  So, one way to get around this when going through an ISA server is to setup a publish rule to listen on 1433 (the default instance for sql) and have it forward the packets over whatever port you are using for the named instance. You must remember though, if doing this not to specify the named instance in your application’s config or else it will try to contact the browser service over 1434.