I’d heard that Dell were offering Ubuntu Linux on some of their laptops and I hadn’t really looked into it. But I came across this page the other day and it’s pretty neat to get Ubuntu (or any Linux) preloaded on a laptop from a major vendor:
http://www.dell.com/content/topics/segtopic.aspx/linux_3x?c=us&cs=19&l=en&s=d
It’s good to support coming from a major vendor, and Dell even offer’s a repo so you can get BIOS updates for support hardware:
http://timony.com/mickzblog/2008/02/05/dell-bios-updates-using-linux/
However, I’m still partial to getting Lenovo’s T400 over a Dell.