I have little doubt that we'll continue to see increasing interest among the malicious fraternity of virus writers in creating more cross-platform malware and attacks. Therefore, when it comes to the heterogeneous computing environment, you'll need to have virus protection on all your machines -- even those that haven't been targeted as often in the past.
Unfortunately, while the number of antivirus choices available for Windows is astounding, the selection for Macs and Linux is sadly lacking. There are versions of the Clam AntiVirus freeware for Linux, Mac and Windows systems. For a mixed Linux, Mac and Windows environment, you can get products from Sophos (which I use on my Mac), McAfee and Symantec that also cover all three. For mixed Windows and Linux environments, there is also Kaspersky Lab, F-Prot, Panda and Grisoft, to name just a few. Some of these are even offered as freeware, which means there's no excuse for running bare.
As for future targets, the Macintosh is likely to see still more action. Adding even more temptation for the virus-writing underworld, the Mac is now using Intel chips. This allowed Apple to come out with software called Boot Camp (www.apple.com/macosx/bootcamp), a free download that allows users to boot their Macintosh computers into either Mac OS X or Windows. I think it won't be long before the architects of digital disease are freely distributing equal opportunity malware that can infect Mac OS X, Windows and Linux -- sometimes zapping a couple of targets on the same piece of hardware.
One final note regarding keeping heterogeneous networks virus-free: malware is often written to take advantage of flaws in programming. Flaws affect every operating system, so regular patching of your Windows, Linux and Mac machines remains paramount. When searching for a patch management tool for your heterogeneous environment, look for one that covers more than one operating system.