Compromised (zombie) machines?
I think you mean Compromised (windows) machines.
Cybercrooks have adapted to the takedown of rogue ISPs by building more resilient botnets. An annual security survey by MessageLabs found that the already high level of spam reached 87.7 per cent of email traffic during 2009, with highs and lows of 90.4 percent in May and 73.3 percent in February respectively. Junk volumes …
Sorry, but I just had to allude to the whole "why write virus code for a low percentage population" argument. Personally, an apache-attacking linux virus would be nice to have IMHO, due to high bandwidth and always-on status.
I await the day when Linux (or OSX heaven forbid) take 80+% market share and Windows is able to take the Apple-stance of "Look at me! No viruses to worry about! [because we're insignificant]"