I was thinking that Apple dropped the modem from its Powerbook line, but apparently, it wasn't until after the Intel switch that the modems disappeared. I don't think I ever used the modem in my early Macs.
The iBooks definitely had them. Back on dialup, you could hook up your airport base station to the modem and initiate a connection as needed, but it was just as easy to plug the phone jack straight into your laptop.