The userbase has changed, and the market share is expanding, expect to see unix based viruses (actually expect to see more cross platform viruses).
The equation goes something like this:
( (N - 2P ) / C ) * M = T
Where T is Virus Threat Level.
N is number of numpties using the operating system.
P is number of IT professionals (those who can code assembly :) )
C is choice weighting ( what level of choice of software is there on the system and the actual variety used )
M is market share
I call this the Alpha Numpty Equation for Determining Virus Threat Level.
Now let's add some figures:
Assume a population of 100M.
( ( 79M - 3 ) / 0.5 ) * 0.80 = 126,400,000
( ( 10M - 1M ) / 4 ) * 0.20 = 450, 000
So you can see it is not just not worthwhile at the moment to write viruses for the Linux platform. But the numpties are on the rise in Linux, and more pros are lost to Plan9 everyday now.
The point is Dave, Linux distros are vulnerable, proof of concept viruses are good enough to prove that, but the reason it doesn't get hit is mainly market share and user base.
If you happen to live in a nice neighbourhood, full of builders, architects, gardeners, internal designers, then your chance of being shot is quite low, but it doesn't mean bullets bounce of you. And if the neighbourhood changes; starts to host rock concerts, opens a lap dancing club or two, a few fast food joints turn up, then the chance of being shot increases. Thanks Ubuntu :)