Ars Technica had a good write up on the problems Dell or another OEM might face when trying to put linux on a desktop.

http://arstechnica.com/news.ars/post/20070228-8941.html

One thing that I noticed wasn't mentioned was the political issues that sometimes come up amongst the different linux distros, especially in regards to open drivers. Since Dell would want a distro that's 100% compatable with it's hardware (I presume) then they might run into some problems along that front.