Here's some more data from my yellow lid'ed dreamcast,
- it was manufactured in may of 2000,
- only the exposed surfaces of the lid are yellowed; the inside and edges of the lid are still white
-the plastic on the rear of the modem is yellowing, but not the the other sides, just the side with the jack.
-it's been kept in the box more often than not, and may have had a stint (a week, tops) in the sun when i let my nephew borrow it; today i saw that his playstation 2 was in a place where the sun reached it everyday.
here's a pic of the rear of the modem: