In the past, the few mobile phone viruses to have success have done so through propagation via Bluetooth technology.

Previously, malicious programs like the Cabir virus made use of a phone's Bluetooth technology. Once a phone was infected, the virus, via Bluetooth, would "search" for other mobile units where it could spread. However, this new mobile virus called Commwarrior uses both Bluetooth and multimedia messaging services (MMS) to spread itself to other phones.

The virus also runs on the mobile OS environment, Symbian Series 60.

http://www.f-secure.com/v-descs/commwarrior.shtml

it'd be worse if it sent your old messages out to everyone in your phonebook, or somehing like that.

seems that symbianOS is a good platform to write viruses for ;-)