I wouldn't mind them in movie theaters. It would eliminate the annoying rings of people who leave their cell phones on and for those of us who are required to have their cell phone on for work would have a valid excuse as to why we couldn't be reached.
I have to put my phone on vibrate and if I get a call during a movie I have to go outside and call the person back missing part of the movie, which sucks. If they had jammers I wouldn't have to get their call and I've have a valid excuse as to why.
I don't think that just anyone should be allowed to have a jammer though, it's guaranteed people would abuse the jammers.