Dell didn't ask but if they had I would have told them that Ubuntu Linux is a good choice. I have generally refrained from the usual histrionics whenever something Linux-ish makes the news, but I'd would like to voice a couple remarks regarding Dell's Ubuntu announcement.
While I'm always happy to see Linux reaching a wider audience, Dell has been something of a fair weather friend to the open source crowd. There was once some similar fanfare around their decision to offer "bare metal" computers, that is, sold without any pre-installed operating system; unfortunately the bare metal boxes were never easy to find on dell.com and didn't last long. I hope that Dell's is a genuine interest in providing viable alternatives to their customers, rather than a flash-in-the-pan marketing idea that will soon fade. One thing that gives me hope is the option of FreeDOS: FreeDos isn't nearly as sexy as Linux but is entirely consistent with free/not Microsoft alternatives.
I don't want to sound dour, so I'll tip my hat to Dell for choosing Ubuntu. A direct descendant of Debian, Ubuntu is a stable Linux distribution with strong community support and active user forums. I have recommended Ubuntu many times both professionally and personally and it's nice to see Dell agree with me.