What makes autofill not appear as a sync option in iTunes 11?
I figured it out with some help from Jesse at the Ottawa Apple Store:

If your iPhone has iTunes Match turned on, you’re not able to manually manage your music. In order to set how much space you want to use on your device for music, you’ve got to first turn iTunes Match off, set your space requirements in the Autofill section, and then turn iTunes Match back on.

Good times…