I've got a Ubuntu Server 10.10 with 2 network interfaces with a cable plugged into both going to switches on completely different networks with different routers. One network is the 10.1.10.X network with a separate gateway/router - the server has an IP of 10.1.10.50 with the gateway IP of 10.1.10.1. The other interface is 10.2.10.X, IP 10.2.10.50, gateway 10.2.10.1.
All my Mac machines are on the 10.2.10.X network, and all servers on the 10.1.10.X. The ONLY connection between the two is this machine. From a Mac in my office, I CANNOT ping any computer on the 10.1.10.X network except the Ubuntu machine I'm talking about. However, under the Shared column in Finder, I can see every server on the other network listed. That makes me believe that somehow this Ubuntu machine is letting certain requests span both networks, which is a security problem.
Hope this is enough info.