Opera 10.0 on OS X remembers the non-default country specific google TLD the first time Google redirects it to such a TLD. Having recently visited Australia, my Google search from the toolbar was stuck on google.com.au; clearing cookies and private data didn't help with that.
The culprit is the following line in ~/Library/Preferences/Opera Preferences:
$ cd ~/Library/Preferences/Opera\ Preferences/
$ grep -r 'TLD Default' *
operaprefs.ini:Show Default Browser Dialog=0
operaprefs.ini:Google TLD Default=.google.com.au
All it takes to restore the default google TLD (the US one, in my case) is to quit Opera and remove the highlighted line in operaprefs.ini. Due to this browser's cross-platform nature, it's likely that same approach will work on Windows and Linux. Hopefully it will save some traveller a bit of head scratching.
The ~/ refers to your home folder, it a Unix equivalent of Windows's %USERPROFILE% environment variable.