Tim Bray, former Google employee, currently working at Amazon, writes:
It's an interesting theory for sure, but it seems hard to back this up with any tangible evidence. How would you even test this? You can pick specific web sites to test this with, but that will always be an incredibly small infinitesimally, unbelievably small subset of web sites, and there's no way to extrapolate any of that to the web as a whole. To make matters worse, Google tailors search results to the information they have on you, making this even harder.