Ubuntu is highly popular, and is growing very quickly. Moreover, it cares more about enabling nontechnical users (doctors, lawyers, etc) to get work done than

Ubuntu is highly popular, and is growing very quickly. Moreover, it cares more about enabling nontechnical users (doctors, lawyers, etc) to get work done than any other distribution I've seen. Most of the advances in Ubuntu I've seen have been in the direction of making it easier and easier for people who don't know what an operating system is to be helped by Linux. (cf. Windows migration tools that automatically create a dual boot system, and crawl the Windows partition to find bookmarks, email contacts, etc). As such, I think it's the most suitable distribution for a mainstream vendor to adopt for sale to mainstream users.