When the SE's index sites like whatismyip.com that display the IP of the visitor, the spider will usually also store it's own IP in the results for the index
From there you can just hit arin with that IP to see how much of the block the company owns.
You could also use it to determine whos results smaller engines primarily use.
Example:
http://216.239.57.104/search?q=cache...is+my+ip&hl=en
Then:
http://ws.arin.net/cgi-bin/whois.pl?...t=64.68.82.201