One has to ask as to whether hackers would be interested in attacking Windows 2003 or WM5. Wouldnt smartphones be the best target, transmitting the virus to other phones, either thru bluetooth or WiFi? Since PDA's are moving aside to smartphones and iphones, I would think that the new targets would be WM6, Symbian, Palm, Blackberries or anything other than WM5 or below. By the way Symantec does have an antivirus for PPC.
Symantec AntiVirus for Handhelds: Overview - Symantec Corp.