So, for the sake of argument, let’s take it as a given that this sort of thing [malware on Macs] is becoming more common. What can Apple do? Think about it. (My guess: think about why the iPhone and iPad, despite being far more popular than the Mac, have no trojan horses.)
That’s a very clever solution, as I assume he is talking about App Stores. If users are trained that where they get apps without worries is from an Apple App Store, then they will be leery about letting any other app install itself.
(An added bonus also being that the user iTunes account password is different than the Mac admin password. Thus users see the differentiation in the installation methods and become less willing to install outside of the App Store. Of course that is assuming a lot on the users end.)
UPDATE: As pointed out on Twitter Gruber could be referring to many things, including sandboxing and the like.