Garmin have brought out a small device which transmits a short range message to compatible a GPSr (currently only the Oregon and Dakota).

Apparently it transmits a local message to these devices to offer information, eg coordinates or hints. It would be possible to set a puzzle cache with vague coordinates and leave it to the chirp to guide in cachers (or at least the few with compatible a GPSr).

Garmin Chirp

No news so far if it will be launched in the UK in addition to the US, but I have a nasty suspicion that the UK radio transmission laws may prevent this, depending on the mechanism it uses to transmit to the GPSr.