tedster - 7:17 pm on Sep 27, 2012 (gmt 0)
How can they turn it off for some types of search but leave it on for others?
Google already does this in many different ways. It's somewhat that same idea as ad retargetting.
This ability to change SERPs on a dime for an indivdual user is also part of the reason why ranking reports can be such a total puzzle. If a user showed such-and-such a previous behavior (either cookie based or logged in history) then the SERPs they see can be quite different.
Consider this. We know that Google has automated taxonomy creation - both for query types and for web page types. If someone's previous behavior suggest they are looking for information, then why wouldn't Google stack their next SERP with more informational choices rather than transactional ecommerce pages or navigational choices. Their only limit would be doing the programming and data storage.