I have to say that this problem affects me, too. I have 3 Macs, one Dell.
The Mac running 10.3.9 can see the Dell.
The Mac running 10.4.11 can see the Dell.
The Mac running 10.5.3 can only see the Dell if I go to "Connect to server" and enter the Dell's IP address. This seems like a giant step backwards if Panther can find the Dell, but Leopard shows me nothing in the sidebar (well, except for the other Macs).
Can anyone offer suggestions? I can find no real solution anywhere.