ICSA Labs have published their 8thComputer Virus Prevalence Survey. The survey found that the growth rate of malicious code infection has slowed. There also appeared to be a move from "Bug Bang" (for example, Code Red) events to persistent threats (such as Klez).
Updated: 21st November 2007
The full survey was formerly available at http://www.icsalabs.com/2002avpsurvey/index.shtml