I'd be alarmed if it were possible to activate the GPS remotely, although it would be a good feature to have in the OS - say you can enter a list of emergency numbers (or maybe hardcode them in if practical), and if you dial any of these numbers via a cell connection the GPS is automatically activated.

Or if there is some signal that can be sent over the cell network to request GPS coordinates, it would be good to have a setting where the user can allow/disallow this. But I sure wouldn't want my phone giving away coordinates willy-nilly.