There is a PPC virus out there:
http://www.pocketpcfaq.com/faqs/virus/virus.htm
There is another one which is simply a "proof of concept" virus. Don't expect yourself to be completely vulnerable to the whims of some script kiddie, as the word macros and so on will not run on the PPC (as far as I know, anyone care to correct me?)
You can still download PC executables and scripts and so forth that harbour virii designed for the PC that will do no harm to your PPC but will make your PC life miserable...