I’ve solved the problem with the printer shared via USB on a Windows XP box! I can now print to it from my Vista box.
I found the solution at TechArena. You can find it here. Basically, you need to install the printer locally first, then fake it out using a Port that points directly to the shared printer. Then, be sure to disable the “Offline Printer” support so that it prints over the network immediately.
UPDATE: And here’s another post, which makes things a bit more clear.
