Both of my parents are using Linux exclusively. I use Linux exclusively at home on all of my machines except for a separate Windows VMware guest image that I only use when I need to work from home. None of my computers boot into anything except Fedora.
I make all of my software and hardware purchase decisions based upon Linux compatibility. That means I'm a much less active gamer than I used to be although in part because although there are some good gaming options available for Linux, such as Quake Wars, the overall Linux gaming market is still limited.
I switched completely to Linux because at some point the dual-boot option becomes really annoying. When you can accomplish 80% of your normal tasks on Linux it just seems much less worthwhile to maintain a separate OS that you need to boot into for that last 20% and eventually I found substitutes for that last 20% as well.
I partially agree with your point that many people who run Linux are also running Windows in their home or at work but I actually think that more people could run just Linux and not lose any functionality. My parents really have no desire to do anything beyond web browsing, e-mail, and basic web media with their computer. But then again they probably aren't the best target group for porting most software to Linux because they probably aren't going to have any need to purchase it (except Quicken).
> time.pl
1234567889 (Fri Feb 13 23:31:29 2009)
> timepl.
timepl.: Command not found.
> time.pl
1234567893 (Fri Feb 13 23:31:33 2009)
Stupid typo!
...though his invention worked superbly -- his theory was a crock of sewage from beginning to end. -- Vernor Vinge, "The Peace War"