As of 4/11/06 I am unaware of any viruses in the wild for Microsoft Windows Mobile. Known viruses for the Symbian OS require the operator to make the mistake of downloading and accepting a strange file. If you are still worried (not necessarily a bad thing) there are lots of companies (odd, considering the lack of threat) willing to take your money for their protection. Do a Google for Cingular 8125 antivirus and check out the options. I have no recommendation since I have never used this sort of app.
How can I get virus protection for my Cingular 8125 pc phone when I connect at the local wireless hot spot.