Apple currently sell the Belkin Infrared adapter on their web Apple Store sites, but loads of people have had trouble using it with Mac OS X as it includes a floppy disk with PC drivers on it. After reading loads of sites, and seeing everyone having the same problem, I thought I'd better post the answer I have found...
First, plug the SmartBeam into USB port one on your PowerBook G4 (that's the one closest to the power socket). Turn on your infrared device, and activate its infrared dectection and place it in range of the smartbeam. Now load the Apple System profiler by clicking "About This Mac" in the Apple menu and then the "More Info" button (or via the Applications -> Utilities folder). Click on the "Volumes & Devices" tab and wait for it to load then select "Window" and then the "Refresh" option. The IrDA adapter should now show up as a USB connection.
Load the System Prefs and select Networking, and this should force Mac OS X to find a new Comms Port and it will load IrDA USB Modem into the available options!
Hope this helps all the lost people out there took me a week to crack it -- it worked on PB 12" G4 Superdrive running 10.2.6.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003072008542035