Slashdot had a link to this article on a new service in the UK called BlueCasting. This is a brilliant idea. Embed a directional transmitter into a billboard that offers to send additional information or promotions to cell phones that are in viewing distance of the advertisement.
The range of 100 meters seems a bit excessive to me. It may be fine for the initial rollout, but the airwaves could get quite crowded with promotions if the idea took off.
What if, instead, the range were limited to something like five to ten meters. Just enough so that you receive a small discount on the newest Britney Spears CD as you passed by your local music store--but only if you acted in the next hour.
Of course, the system would also need a way to limit the number of times it interrupted you to keep it as a useful service instead of turning into a new form of spam. Well chosen, personalized ads delivered in real time could be extremely effective.
