Richard Etter designed this cell phone detecting AwareFashion shirt to assist the staff at an opera house. Containing a module that detects activity on a GSM band, it can be used to alert the staff to patrons who have not turned their phone off (and may be in danger of interrupting the proceedings with a cell call).
It appears to be just a concept, however I can see the potential for it being a more polite way to deal with the issue than just blocking all cell reception. This way, the staff is will be able to confront each person individually, which could allow them to make exceptions for people with valid reasons to have a (muted) phone on. Think it could work? [thanks, Val!]