I wonder why the free Find feature is only for the newest devices? Most of the time Apple is pretty good about only locking devices out if their hardware can't handle it. The 3GS should be able to handle it, right?
* 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.
So you can use iPhone 4 to activate the feature on 3GS, though I have not tried yet.