IE (with certain settings on) is sending page data back to Microsoft. If it sends the URL, title and referrer back then the following session is pretty easy to reverse engineer.

    1. URL: "test - Google"
       Title: http://www.google.com/search?q=test 
	   
    2. URL: http://test.org.us
       Title:  "Test"
       Referrer: http://www.google.com/search?q=test 
       Time: 2 secs

    2. URL: http://en.wikipedia.org/wiki/Test_cricket 
       Title:  "Test cricket - Wikipedia, the free encyclopedia"
       Referrer: http://www.google.com/search?q=test 
       Time: 249 secs

It's really just an extension of page rank by seeing what links are being clicked on and not just which links exist. Whether MS should be capturing this data under false pretenses is another issue.