Well actually,
With any iPhone 4 you get free tracking functionality of your phone through
http://me.com -- just log in with your Apple ID. I can track the GPS location of my phone, send a message to it (with out without an annoying noise), lock the phone, change the passcode of the phone, or if I just can't get my phone back, I can WIPE the whole phone. All from
http://me.com
The functionality is available on iPhone 3gs too, but it is not free. You need a Mobile Me account. But with iPhone 4 you get the "Find My iPhone" feature for free...
Apple - MobileMe - Find your iPhone or iPad.
SLLLLUUURRRP