One advert might do it
The biggest blocker to mainstream acceptance is that most people have never heard of Linux, never mind the fact that they are choosing Windows instead. Before the Mac entered the public perception in a big way, most people just assumed that Windows was part of the hardware, a built-in, a given, with no choice. Truth be told, most people still think that, such is the pervasive status quo.
If someone could stump up enough cash to get one or two prime-time US advertising slots at a time when an awful lot of people are watching and with imagination do something that captures the public attention, that might be a good start. It doesn't have to get a big philosophical message across, just get noticed by a lot of people. Get people talking. "What is this Ubuntu thing?" (Yes , why Ubuntu? - just pick a brand that people can latch onto for goodness sake)
Mr Shuttleworth - I'm looking at you :D