The pings happen "every few minutes" according to snopes.

http://www.snopes.com/crime/prevent/cellping.asp

I think that limits the amount of useful information that can be gathered.

That limits the amount of information they cat extract about paths. But you still get a lot of snapshots.

Nano tech will, if it isnt already make this redundant.

Forget 'every few minutes', nano in the carpets are watching your every footstep!

And tomorrow id like my bmw to be blue - wheres the remote?