Robert_Charlton - 5:52 am on Jan 17, 2011 (gmt 0)
One more possibility. Each browser has different cookies, so conceivably each could have a different location preference set.
(Setting is in the left-hand margin of the serps page, under the list of main vertical channels... ie under Images, Videos, News, Shopping, More).
Prior to the introduction of the user-set location preference, I remember that Google would be highly reactive to recent location-based searches, and, if I weren't signed in, would flounder for a search or two after I cleared cookies.