AT&T Cable in San Francisco is an odd beast. Their central will hand out DHCP addresses to any host connecting to a cable modem - rather nice, since you get a mostly-static global IP address for each machine (at least four allowed per household), nor is a "router" required - a hub is plenty enough. Sadly, the IPs they hand out are rarely on the same class C (Subnet Mask 255.255.255.0) network - the network mask their DHCP server hands out! What this means is that Rendezvous will not work between machines in the same house.
The solution is to give the machines a secondary IP address which is on a shared IP subnet. My method is to use the command /sbin/ifconfig en0 alias 192.168.1.?, where ? is a number between 2 and 254 - different for each machine. The 192.168.?.? subnet is "private," which means it won't colide with any global addresses.
This solved the problem instantly. A little bit more work, and I had a startupItems script put together as well (contact me for details) that adds the alias every time the machine boots.
It is VERY cool to be able to share a USB printer between two Macs with nearly no effort.

