Take Offline

Google doesn't do anything evil... Google only wants to give you what you want, or ads for what you want.
If you switch ISPs the rDNS changes, yes. But not the fact that Google has used it as an identifier for you, to map your doings. - If you use google services, they'll peg the new rDNS to that as well - adding, not detracting from their knowledge.
Switch ISPs, and Google not only learns your new DNS, but also that you have changed ISPs.
Use an "anonymizer" and google knows that you use one, since they have a limited number of IPs to them...
Google learns from everything.