Chris Howard is correct - the amount of Linux desktop usage is virtually insignificant.
This shouldn't detract, however, from the massive success Linux has had in other areas.
If you look at statistics from the other side, how many servers on the web are Linux based?
I think the idea that Linux will ever conquer the desktop is a moot point - it has no real importance.
The next "big thing" will be the mobile internet market and this is where Linux will meet John Q Public...
Apple is Killing Linux on the Desktop