A couple of cell phone detector circuits: Cell phone detector with schematic, explanation, and another. Some students last year tried these out with mixed results, might need to be investigated further.
***UPDATE*** found a couple useful links for more general EM detectors. This one looks like a winner. It is referenced in this adafuit blog post.
********UPDATE 2*********An EMF detector from Aaron ALAI and one from MAKE EMF Detector
Plus one more.