I have been using Macs since they first came out, and have only heard about three viruses - all benign - that were directed at Macs.
My XP machine is under constant attack and it is not only the software like Outlook and IE that is targeted, it is the OS as well. Friends who write code for a living tell me that the MS software coding is 'dirty' - really easy to attack. MS is constantly busy writing patches and fixes ("service packs") instead of writing the next great OS.
Mac already has, with OS X - Panther, but as it is Linux based, it may be more open to attacks in the future. Thanks for the warning. I'll keep a sharper eye on protecting it.