AIX system can easily be a "Virus shelter."
We use samba to share out folders to our intel users and it did not take them long to realize they could store documents in the AIX folders as well as their other NT / Novell provided shares. We use virus scanning as much (or more) than the next company, but we found that folks kept getting the same virii over and over. Turns out our network guys are pretty anti-Unix and in their shortsightedness it created a blind spot in our virus scanning strategy because they had not even considered scanning the samba shares until I pointed them out and of course thats where we had some infections hiding. These were pretty run of the mill windows type virii and were thus pretty inoculous to the AIX server, but I guess to the admins who are primarily concerned with the more "popular OS" looking into your unix/aix shares would be an easy oversight.
But, to the orginal question, with IBM's Linux affinity stratgy for AIX and that OS's popularity with VX'ers,I think its only a matter of time before we see an AIX virus. AIX's lack of virus threats for so long may have created a blindspot of its own for AIX admins and so we will probably (knock on wood) all get totally nuked one of these days.
I am surprized some enterprizing startup hasnt released some virii wildlife for AIX just so they could sell a solution.