As long as something has write access, it can be infected.

Apple's distributing its next OS via iTunes. MS probably will do something similar eventually (and they already do their "Anytime Upgrade" that way, I think). I suspect at some point in the not too distant future OSes and the partitions on which they live will become read-only. You need an OS update? You flash a new ROM area on your main Flash storage. It won't get rid of viruses that are only RAM based, but it'll prevent the bare OS from being a vector. User files and programs could still be infected, of course.

This will work as long as the OS vendor is virus-free.... :-/

Cheers,
Scott.