How about Dell?

Dell is shipping the Mini with Ubuntu installed, customized by them. Acually, they seem to have hired Canonical, given that Canonical is hosting a repo for them: http://dell-mini.archive.canonical.com/ (I may be wrong).

And the thing is: Minis with Ubuntu cost less than with XP (not much less though: $16 less as of today, but it makes sense in terms of the cost of an XP OEM license and the cost of Linux customization). So, this can be done, and it can be done properly.

The upshot for the OEM is not so much how much money they can shave off the price, but how much and and how well they can customize the produce, in my opinion. The downside is always the same, Windows is a familiar product for users. But that didn't prevent Apple from selling millions of iPhones, or Asus from selling millions of Linux based eeepc's at the beginning of the netbook revolution.