Such devices are illegal in the US.
I would like to see a standard for a specialized signal that would instruct cell phones that were equipped and set to receive it not to ring (the cell phone could be set for whether to allow vibrate or not), or could be set to not ring but vibrate when a message was received, etc. People who had a bona fide legitimate need to use their cell phones under such circumstances (e.g. doctors receiving emergency calls) could set their phones to override mode, but anyone whose phone was set to override mode would risk the social stigma associated therewith.
Such a signal would have the advantage over jammers that (1) it would not prevent people from discovering that they had received calls or messages, and (2) it would not prevent people from placing calls in case of emergency.
Another possibility would be to "tinfoil" the courtroom - make it into a Faraday cage.