After spending a day fiddling around with ports and Bonjour to get a USB printer attached to an AirPort Express to print from my XP Desktop, I finally figured it out. It was my XP firewall software; I disabled it, and could at last print. The firewall I was using was the Zone Alarm freebie version; I'm now reliant on the XP built-in firewall along with Spybot, AdAware, and Mozilla, but at least I can print.
I hope this helps someone else -- there's a lot of stuff out there about printer configuration, port fiddling, and using Bonjour, but in the end, it was nothing to do with any of that. It was a simple software issue...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051101103117716