Klaus : I have taken apart a commerical unit which picks up signals from the power lines, and there is no transformer or anything - it uses a motorola 86 microprocessor, a bunch of resistors, an open air coil, diodes, a couple of pots and really nothing else unordinary. However I had to return the unit shortly and was unable to dissect the PCB and the contents (the PCB construction would have made it impossible anyways.
Stevez : thanks for the info, I suppose a simple circut to filter out the unwanted areas would do some luck. The unit mentioned is a Pivot Shift-Light X which performs extremely reliably. Afterall, this is a "frequency switch" and not a 'frequency display' so it's probably set to trigger at a certain voltage..