Workbench: Allow users to publish revisions without moderation
This is sort of bizarre and I'd love to hear other answers, but after much trial and error with Rules, it turns out this seems to work with the following setup:

Event: After updating existing content [so it's not a new node]

Condition: Content is published [so admin has approved it at least once]

Reaction: 
1. Publish Content
2. Set moderation state during save: Published

I would have thought that either #1 or #2 would have been sufficient, but strangely if they are not combined, it always reverts to draft. Also strange, the order doesn't seem to matter. I've had it working #1 then #2 and #2 then #1.