When Apple announced Find my iPhone was going free this morning they seemed to restrict it to 2010 hardware: iPhone 4, iPod touch 4, and iPad, but now it looks like there may be a way to get it working on iPhone 3GS and other unsupported devices. In fact, Apple.com tells you how:
Create a free Find My iPhone account on any iPhone 4, iPad, or iPod touch (4th generation) running iOS 4.2. Once you create an account on a qualifying device, use your Apple ID and password to enable Find My iPhone on your other devices running iOS 4.2. Find My iPhone is not available in all countries.