Here's links to two interesting virus-related papers released recently:

http://www.dtc.umn.edu/weis2004/weaver.pdf
"A worst case worm", by Nicholas Weaver and Vern Paxson (and Stuart Staniford) - these are some of most respected researchers on worm spreading theory.

In this paper they speculate that a worst-case worm could cause $50 billion or more in damage by attacking Microsoft Windows systems and carrying a destructive payload.

http://policy.gmu.edu/imp/research/Microsoft_Threat.pdf
"Is Microsoft a threat to National Security? The effect of technology monocultures on critical infrastructure", by a Gorman, Kulkarni, Schintler and Stough at the George Mason University.

To balance the previous paper, this research shows that while Windows worms can be really expensive, they don't necessarily take down the world.