The general concensus at the moment is that there are no appreciable ppc
threats and the problems and performance loss by running one of the few
av programs is not worth the effort.
In general, the ppc is a "client" device and is, therefore, not subject
to the same threats as the desktop.
The biggest threat is between the ears of the user... if you are wont to
download and allow a malicious ppc program to run, install ppc av
software immediately
Beverly Howard [MS MVP-Mobile Devices]