I know this is old, but since no one responded I thought I should...I'm part telecom, part IT.

Pretty much your only choice for a laptop with a built in modem and serial port is going to be the Panasonic Toughbook 53. The other 2 Toughbook models have serial ports as well, but not a modem.

The upside of this is that it is a very sturdy, reliable laptop. It is meant for field work, and can survive some water and drops. The downside is that they are not the greatest performing laptops, and they are expensive. Starting price is about $1300.

I used to work with the Anhsuer Busch distributors supporting their mobile sales system. The Toughbook was heavily used across the country. Good little machine, and I only recall ever sending 1 in for repair.

You won't get a serial port or modem from Dell or HP at all.