This is the best comment I've seen so far on when Linux Desktop will start to gain momentum. (Hint: It has very little to do with X, KDE or Gnome).
Word of mouth.
Really, Linux isn't that unknown. Especially when all the geeks/nerds/admins are running around with Linux orientated Con shirts and laptops with Ubuntu or Fedora on it. I have converted my share. All have been told I can reinstall windows for them for free if things don't work out. Zero requests to do so. Most have been followed up with too.
And thats just me.
More companies are using it. Schools are starting to switch. Some people do notice this, and spark interest. Just pretend now you are a tech noob, and hear about this Ubuntu thing. What do you get when you google that? One of the better Linux homepages IMHO. Enough to at least spark interest or curiousity. And most people have their resident geek to ask about. Thats how over half of the people I know are on Linux (mostly Linspire and Ubuntu) found out about it. Some blurb somewhere and drilled me.
However, that being said, some real advertising beyond IBM is needed. Especially with the context of the IBM ads. They are aimed at us and the PHB and the army of Sr Admins that control what toys get bought and when. When Ubuntu or Linspire get ads with a Dell or Gateway, then the real momentum will begin.