I have a Canon USB printer plugged into my G4 tower and an AirPort network, and I can now print from my powerbook G4 - very exciting!
Setup was a piece of cake: On the machine with the USB printer attached, open System Preferences -> Sharing, and enable printer sharing. Next go to the other AirPort networked machine, open Print Center (in Applications -> Utilities), choose "Add" using Directory Services and Rendezvous options on the drop downs, and there you go, an airport networked USB printer, sweet!
[Editor's note: Actually, Rendezvous is even more amazing than that. On our iBook, all I had to do was select "Print" in any application after enabling printer sharing on the main G4. Without ever visiting the Print Center, the USB connected Epson showed up ... as did our Ethernet connected Brother laserprinter! Both printers had full device information and worked on the first try. Rendezvous is amazing. As an example of just how easy this is in OS X versus OS 9, check out the TIL article on USB printer sharing in OS 9 (thanks, Jacco!).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020831100432677