I don't see how that can be true. A virus is basically just an evil program someone wrote. The only way a BB could never be infected is if nothing could be installed on it. It's not as if a Win Mobile phone just magically is infected by visiting Google.com or sitting near a hacker. The delivery of a virus or worm is the same for all platforms - something is installed that should not be either by someone taking the device or the user installing something they should not.

A virus/worm is just a program or piece of code. Only way to totally prevent one is to never allow any code to be installed.