Why might you wanna do this? Well, for starters, Google Instant does not work everywhere in the world right now. So you can force it to the US-based Google webpage and start using Google Instant. Either temporarily or permanently.
Temporarily : http://www.google.com/webhp?hl=en
You must visit this exact URL each time you want to use the US-based Google.
Permanently : http://www.google.com/ncr
This one will disable the redirection by setting up some cookies in your browser. (ie. You need to visit it again if you change browsers.)
After visiting it once, you will not be redirected to the local Google domain when you visit http://www.google.com