ip addresses are at best unreliable. aol for instance shares a handfull of ip's between 20,000,000 clients.
i would think your best bet may be to think somewhere allong the lines of timezones.
you would need to get the clients time (javascript) and compare it to the server.
though, here again you can see its unreliable as it relies on the clients time.
just my 2c, there must be ways of doing it. hell. when i log in to yahoo, im automatically redirected to there aus site.
someone else? id be interested in this too.