Not having the Squeeze dvd's - I decided the easiest way would be to follow the above instructions. I installed Lenny from the dvd set. After editing the sources.list I immediately received a not found 404 error when I tried the apt-get update. (I did not write down what the sources.list was before editing, just changed lenny to squeeze as suggested in other postings) After searching for a long time I found some examples of what the Squeeze sources.list should have and put that in. Things went smoothly for awhile ~20 hours. Then after the final reboot the mouse pointer became a permanent busy indicator with the display doing a blink every second. Something about the aptitude full-upgrade caused it to break, imho.
So I ordered a set of Squeeze dvd's and started over. Took about 2 hours and it's working. The following from Craigevil is an even better resource for a sources.list:
http://forums.debian.net/viewtopic.php?f=20&t=51000&p=293332&hilit=%7Eknuta#p293332