They actually are cell-phone disturbing devices. In France i have heard about because cell-phones suck in cinema, opera and other live performances.
Then, i think there's already aa patent on this ... so don't say "hurrah i'm going to be rich".
About law : if you use such a device, then it certainly has not to disturb cops frequencies, it may have a limited emitting power (to prevent blackouts over the city, body damages by RF waves...), and you'll have to put a big panel which explain cellphones won't work and also have to ask an authorization.
edit : also you can disturb pacemakers with RF, so be carefull, really
And, about everything that uses RF, never use it with high currents/voltages. RF are also used to transform gaz into plasma... that's harmfull. If you feel nuts : http://jlnlabs.online.fr/plasma/gmrtst/ .