I noticed today that it was impossible for me to type in the full web address for google.com without being redirected to google.co.uk. I'm in the UK, so this makes sense, but before I was always able to visit either site. (There are often more beta features available on the .com site.)
I notice also that there is a link on the UK site to visit the .com one, which has an address of the form google.com/ncr - this alone will get me to google.com (though the 'ncr' suffix doesn't appear in the address bar. Frames? Cookies?).
What's more, I noticed that typing just "google" into Firefox 1.0 adds to the address, so it becomes google.com/webhp?sourceid=firefox - this does not happen in Opera 7.54u2.