A few of the USB dongles work.I remember the Dlink stuff being touch and go depending on the version of the OS. I would try picking up one of those and trying it out.
I know that MacDoc advertises a solution for all USB Macs (running OSX) on his website. Since this is intended for people looking for the original Airport Card I would assume that USB 2.0 is not required.
There's also the option of using an ethernet adapter. I did some looking a while ago and it seemed okay, but only if you're good with a semi-mobile solution.
Powered from the iBook, but still an external device.
You'll have to Google for details as I didn't save the links.
I just bought a D-Link G132 Usb router...It's optimized for USB 2.0 but says that it will work with usb 1.1 but there will be a performance hit. Anyway, it's not explicitly supported for OSX but I'll give her a go and let you know the result.
I know the D-Link G122 was previously supported, and I think you can still get them at Tigerdirect.