Is it possible to detect cellphone IDs in an enclosed area
See "IMSI catcher": http://en.wikipedia.org/wiki/IMSI-catcher

The legality of this varies, and I believe there are more passive versions available. Certainly there are companies offering this as a service (for use in shopping centres to get customer information, for example).

Building your own may be possible with software-defined-radio and "OpenBTS".