Wait...what, as opposed to an edge connection?
You'll need some type of connection to the internet on your iPhone for that function to work. Clearly your adversary won't hop onto every open wifi connection found enroute to their hideout, so in order to make the most of the FMiP feature you'd be advised to get a tower based data plan be it Edge or 3g. If not, your phone will have no way to communicate with the mobileme service and vice versa unless connected via wifi.