03-16-2010, 03:26 PM
Apple sells a USB modem for about $50, or did when I bought my MBP anyway. I use it frequently.
Apple stopped selling that USB modem over a year ago. There are few makers with USB modems on the market that will work. The other option if you have an external modem already is a USB to serial adapter. Those are easiest to find for the 9 and 25 pin RS232 serial ports, not so easy for modems using the Mac serial port connection.