Evidence, do a news group search for "linux desktop" on a year by year basis.
1994 -> 9
1995 -> 38
1996 -> 100
1997 -> 136
1998 -> 696
1999 -> 1,110
2000 -> 1,430
2001 -> 3,620
2002 -> 3,660 (so far)
As you can see, Linux is gradually growing in 'buzz', this, of course, does not translate directly into user growth but it us a indicator of how much Linux is on the conscience of the users of the web.
Five years ago *nix was in the realm of the (excuse the phrase) "ultra geeks". Today, webmasters and SEOs commonly chose linux as their operating system of choice. What will it be five years from now? I think it is wide open.
I don't see the numbers atrophying, there is no way Linux is going to lose ground, IMO the question is, "how fast will it gain popularity?"
Littleman's Nostradamus like prediction of the evolution of open source usage:
Stage I ->Hardcore programmer
Stage II ->CS majors, adventurous Webmasters, the curious
Stage III ->Academia, wannabe hippies types, government, many web professionals
Stage IV -> A full on horse race between Open Source and Closed Source software for market percentage
Stage V -> Closed Source lost 90% of the mass market software space
Right now, we are in between stage two and stage three.
My prediction is that we will be at stage V by 2017, if you step back, you could see it in motion now.