The Register: At the SMX West Conference, Google's Bryan Horling revealed that up to 20 percent of Google searches are personalized based on the user's particular location, web history, or online contacts. For example, if you search for bus schedules, you'll see bus schedules for your particular city first.
However, Horling says that Google tries to keep the changes minor for two (somewhat contradictory) reasons: "For one, when we have information related to the user, it tends to be pretty clear what they're looking for. And the second is that we're not always right. We're inferring information or trying to fill in the context, but we don't always get it right."