It was bound to happen sooner or later. While viruses, trojans and other malicious software are very uncommon for smartphones, the fact that they exist leads many to believe the problem may get worse. Trend Micro has taken a very proactive approach to this, and has designed a software smartphone firewall, able to run on Windows Mobile 5 and Symbian 9.1. The increased volume of phones, especially PDAs, leads them and many others to believe that devices running those OS'es will be targeted more often:

"IDC estimates that over 100 million smartphones will ship this year," says Trend's device security director Todd Thiemann, "and viruses need the ability to run processes in the background, so even Java-capable basic phones aren't a big threat."
The more complex phone software becomes, the more places malicious people will have to explore for exploits. Does it really have the potential to become that bad? Should people really be expected to buy additional software for their phone just to protect it? It seems to me that the burden should be placed on the manufacturers of the phones and the operating systems they use, instead of a third party.