I believe that regular users (i.e., non-developers) are unlikely to start using Linux on the desktop until they can get it pre-installed. The reality is that all OS upgrades are a nightmare. Lots of people had horrible failures when trying to upgrade from XP to Vista, and even the Tiger to Leopard upgrade on OS X required many users to reformat their hard drives. Given that Linux is a completely independent OS (and, unlike OS X, needs to work across a wide range of hardware), it’s no surprise that upgrading to Linux is so (deservedl) daunting.
That’s why I’m excited about news about the new “tweener” category of mini-laptops coming pre-installed with Linux, such as this article about Acer.