The problem is not people carrying cell phones into theaters. It's when the cell phones in theaters ring audibly, disburbing other people. The audible ringer is the problem, not the whole phone. So if you ask me, the real solution to this is not to install cell phone jammers in theaters, it's to install cell phone ringer silencers.
The idea being, when you walk into the movie theater, your cell phone notices that some radio signal in the air is telling it that this is a "quiet zone." Then it can switch itself to vibrate mode automatically. This way, people can still carry their cell phones around everywhere, including into movie theaters and plays and fancy restaurants, but not have to go through the trouble of silencing their phone when they walk in and unsilencing it when they walk out.
Of course, this will require the cell phone makers to incorporate such a feature into cell phones. But it shouldn't be too hard. Cell phones are nearly constantly receiving radio signals from nearby cell phone towers anyway, so having them check another bit in the packet shouldn't be too hard. I mean, if they're integrating digital cameras and MP3 players and over-the-air video watching into cell phones, how hard can this be?
Of course, there should also be a setting in the phone to disable this auto-silence feature. I'm not sure who exactly needs that feature, since a vibrating cell phone is probably more annoying to the person whose pocket it's in than a ringing one. But I'm sure there's some reason the FCC will demand it. (Just like how the FCC demands that movie theater owners not install cell phone jammers in their theaters now, because a police officer or fireman inside the theater might not be able to answer his cell phone. :P) Now, there will certainly be people who flip this setting. Their phones will continue to ring in the middle of movies, and disturb everyone else. But with a lot fewer of them, and a feature already in thier phone to prevent it from happening, we can feel a lot better about getting in their faces and telling them that they're rude, inconsiderate dorks with room temperature IQs.
As for me personally, I can't think of a reason I'd want to turn the feature. I've forgotten to turn my ringer back on after coming out of a movie several times, and I would love something that would automagically turn the silent mode on and off when I entered a movie theater, or play, or fancy restaurant. To me this is like the remote auto-lock on my car's keychain. Yes, I COULD lock all my car doors by hand. But I don't have to, so why go through the hassle when there's a better way right there under my nose? The mechanical locks are still there in case I need them, and that's a good thing. But having my choice of which way to do things is even better. More options is good, and this gives me more options.
And yeah, I'm sure that sooner or later some prankster will figure out the protocol and make a backback-sized mobile cell phone silencing station. Fine, let 'em. We have exactly the same problem with pirate FM radio stations. That's one thing the FCC is actually good for: To track down people who abuse the radio spectrum, confiscate their equipment, fine them, and if their actions have hurt anyone toss 'em in jail.