A very pleasant surprise today. Thanks to the hard work of Marc Boucher and Ben Herrenschmidt, there's now a Linux driver for the iBook2's internal modem (has some binary portions). This means I can stop carting around the external USB modem with me everywhere.
Kudos to Marc and Ben, and many thanks for helping me out with debugging my setup. The drivers are here. I also needed the 2.4.20-pre5-ben0 kernel to get things going OK.