The inspiron 8600, 8500 and 600m have serial ports.  The 5100 series may have them, but i didn't check. You can also look at the latitude line of notebooks as well.  The Latitude d800 and d600 have serial ports.  You can look at the specifiications of each computer and what ports they offer on dell's website.  Select a particular model you're looking at and hit the tab for product views.

The 5100, 1100, and 5150 don't have serial ports. The 8500, 8600, 500M, and 600M have it. I would personally go with either the 500M or 600M