I'm not even clear on whether ATT lets subscribers opt out. Does the offer say you can opt out of the wiretapping or that you can opt out of only the targeted ads? If you care about privacy, the DPI is the enemy, and the ad-targeting is irrelevant (you already have the ability to not retrieve ads, just blacklist the ad servers). But if what ATT offers for the higher price is only to not target ads, they'll trick most people into thinking they still have privacy, while the DPI is applied to everyone.

It's the difference between tracking only by server/client tricks (IP, cookies, hit logs etc.) vs. tracking by the ISP. The former can be avoided by client config, the latter only with a VPN.

Even if they let you supposedly opt out of the DPI, who trusts a corporation like this? And as adestefan notes, where do people get the assumption that Google is not doing the same?

Edit: I finally found ATT's (semi-) disclosure about this: http://www.att.com/esupport/article.jsp?sid=KB421828&cv=812

It says that if you don't opt out they collect " webpages you visit, the time you spend on each, the links or ads you see and follow, and the search terms you enter" - but there is no statement that they're not collecting that info if you opt out. All they promise for opting out is not targeting ads with that data.