The funny part about this claim is that 90+% of Internet users use DNS solely to get to Google. It's really a choice between having one big company tracking your habits, or two, one of which is small, under intense pressure, and hungry.
E.g., if they want to get to Dell's website, rather than typing "dell.com" into the URL field, they type "dell" or "dell computer" into the search box and then click on the first result.
So a reputable DNS service that did this automatically would be a direct threat to Google.
User A: Google DNS: search for dell, dns lookups; dell.com, chat1.us.dell.com, checkout.dell.com
User B: Non-Google DNS: search for dell
The point I'm making is, it doesn't matter if people use Google for DNS, because they're already handing Google more valuable information. But it does matter if they're handing it to OpenDNS.