Tim from Sound Support was two hours into diagnosing my networking issues, and his imperturbability was starting to fade. "It's not supposed to work like this," he said, sounding almost frustrated. "I get that a lot," I said. "That's why you're here."
After a lot of complex deductive reasoning and many Unix command lines later, we found the problem to be an obscure setting in the computer ownership file. Seems that when I first got my MacBook Pro, I had messed something up as I was figuring out how to navigate the new OS (at one point my home folder vanished, and I had to retrieve it from wherever I had accidentally moved it--this might have been the instigating event). Since had I cloned all my settings from one computer to the other, both had the same problem.
Wireless networking still doesn't work, and won't until I get a wireless router that doesn't issue its own DHCP commands in conflict with the DSL modem's. I still don't like that networking on a Mac is a manual operation--you have to direct one computer to see the other every time you restart the computer, where with Windows it's a one time event. The drives on one Windows machine are always visible on another, once you map the drives. But it appears to Just Work now.