Silvery - 10:07 pm on Jan 12, 2011 (gmt 0)
I've seen no sign of a radomiser, per se, but other factors also figure into getting differentiated results.
If they're all on the same computer, each browser still reflects the same IP address.
In addition, Google has multiple data centers, each of which will have slight differences from one another at any given time. Google has seemed to sometimes assign different browser users to different data centers when delivering up results.
Finally, they sometimes may test different results, differentiating by browser type as well as other unique user factors, so you could be getting assigned some different results based upon them testing something.