An update, FYI: I've since found that I can change, willy-nilly, which two accounts have access to "As items arrive" by taking one of the ones that currently has it OFF of it and down to a longer option. It'll then make the option available to all other accounts until I set it, at which point...
So I remember noticing something like this in the previews, but now that I'm running RTM it's still there and bugging me:
SOME of my Gmail accounts seems to connect via an appropriate protocol that supports Push, adding "as items arrive" to the list of download frequency, and removing the...