How to get free Find my iPhone on iPhone 3GS and other unsupported devices

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.

[Find my iPhone page via @jsnell]