OS X's half-white-list mode of refusing to run unsigned code unless you invoke it from the right-click menu seems to be incredibly effective.
Between code-signing and sand-boxing, I see virus scanners as failed legacies of the past. They have stopped little, and cost everyone greatly.
An independent authority (or authorities) of trusted (and untrusted) signing certificates.
In a word: reputation.
Nothing is 100% accurate in security. But code-signing is still far more protective than virus scanners. Given evading a virus scanner and evading code-signing, one of these is far easier than the other.
When the iPad first came out, a few executives publicly said "this is so awesome, I can do all my work on my iPad!" Sounds good to me, leave the power tools to the grown-ups who can resist punching the monkey, and can install security updates every couple of days (without being forced to reboot at the whim of the OS).
If many people really need to be protected from themselves (and as a side effect save the world from botnets), iOS is the way to do it.
