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I am sorry to say but although there are locating applications available for the iPhone, They would have to be loaded onto the phone prior to misplacing it so you could use them.
If you do not have a locator or Mobile Me account already setup on the phone then I am really sorry but there is nothing you can do with software or GPS that can help you locate the phone.
If you do have a Mobile Me account setup on the phone then can you go to http://www.me.com/account and login and you should then have the option to find your phone, At which point you will then have the option to Map your phones location.
Let me know if you need more info or clarification.