I've been involved in negotiations with a couple of contracts relating to Google Apps for Enterprise/Education.
In each one, the "scanning" has been explicitly mentioned in the contract. In each one, scanning for the purposes of advertising has only happened if the domain administrator allows it to happen. If it is turned off, Google will not scan mail for the purposes of advertising content.
There are of course other reasons why google will scan your email. Spam/Antivirus filtering and indexing to enable search functionality are two that come to mind.
Basically, all Google have done is remove the domain administrators ability to allow ads, and I'm not aware of anyone I know who used Google Apps for Education/Enterprise with it turned on anyway.