Msg#: 3916298 posted 6:32 pm on May 19, 2009 (gmt 0)
Google has been doing this off and on for a long time. It's apparently one way that they split their various tests over the market to gather data.
Another reason this might happen is if the searcher is logged into a Google account on one browser and not on another. This situation means that the logged in browser will see personalized search results.