Apple doesn't need anti-virus software, they just need an App Store for macs, where all software is linked to and signed by the author. This makes the author accountable. Apple has already incubated an "App Store Culture" so it wouldn't be a hard sell. OS X could sandbox non-app store apps and locally compiled apps and allow the user to assign a level of trust and or privileges to each. The default security level would allow only app store apps.
I haven't really put a lot of thought into this but it seems like a good idea on the surface. There wouldn't be a need to jailbreak a mac - you could just change the security level to allow non app store apps. It would also fit nicely into Apple's control-freak psycho personality. Just a thought.