I don't think signal strength is going to be sufficient as antennas are not perfectly uniform point sources - orrientation will matter. Of course, you could use accelerometers in the mobile unit and broadcast orrientation, too. getting kind of complex, though.
frankly the more I think about this more it sounds like a dead-end.
why not look into using motes? search for tinyos. have a number of the receivers sprinkled about. the "mobile item" would broadcast and the sensors would receive and report. you could then do a "centroid" analysis on which motes report receiving the mobile signal and determine a likely location. Maybe even vary the mobile's broadcast strength to better isolate the closest mote. Won't give you exact location.
seems like a lot of work to find lost keys. we still don't know what problem you are trying to solve.