I led a meeting today and viruses came up. I recalled from the FAQ section at blackberry.com that said, "can a virus infect a blackberry"? and the simple answer was no.
It hasn't really reached the mainstream but if ask me it's only a matter of time before hackers, virus writers etc. start going after the mobile computing platforms and smartphones.
Its a basic feature of Java - "sandbox" security model. Blackberry even takes it one step further only allowing signed APIs to access critical system features. This way they could find out where and who provided it.