I agree 100% that various levels of opting out should always be possible.
Some of that is available now - e.g. incognito mode in Chrome (or the equivalent) or simply not being logged in, Search+ has the toggle for whether or not to include personalized results are the examples that come to my mind first. One could imagine other controls that might be useful or desirable.
"But I don't want it to take my entire online life into account when I am doing a search"
Personally I think that over time, the difference in the quality of the results provided by search engines that don't know anything about it's users will so much behind those from search engines that do that we will look back and wonder how we ever found anything.
But I don't want it to take my entire online life into account when I am doing a search. (At the very least I should be able to opt out of this).