If there were such a way to achieve the number of persons tuned into a given frequency (over the air, via standard AM/FM frequencies) trust me-
Arbiton (or whatever ratings service is applicable in your neck of the woods) would have jumped on that band wagon the second it was tested & proved.
Of course, satellite radio and cable tv & radio is a different story, especially with addressable scramble boxes, they can tell which subscribers are tuned into what, for how long, and a lot of other data you would be surprised that is collected, all of course stored and dissected to help "Improve" the quality of your listening and viewing experience

(yea ,right I'm buying that line of hype)
Now, for those of you who may muse about what may be coming down the pike with regards to the radio broadcast industry (in general) and the possibility of actually discerning how many listeners may be tuned into a given frequency at a given time, (and for that matter, even customizing the program content and targeting it specifically to an individual listener, be it in their home, or care, or wherever they may be) I would highly reccomend checking out this increadibly GREAT RADIO PARODY (which is the follow up to the original radio parody, produced by a very eclectic group of radio junkies from back in the 1970's) called: 99!
The original parody called: "9" (The Ultimate Radio Format)
Is the cleverest, most wickedly pointed satire of Top 40 Radio formatics ever produced. Follow the evolution of the fabled WVWA, Poundridge, New York, as its on-air sound evolves and matures.
You can listen to it on line by navigating to the url below;
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http://www.reelradio.com/philpott/index.html#nine
Fast-forward to September 9, 1999 (9/9/99!). Entirely via e-mail, all four now forty-something founding fathers of "NINE!" reunite and produce an update to the original parody called: 99! you can check that out at:
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http://www.reelradio.com/99/
If you are at all a tried & true radio buff, and remember what radio once used to be (back in it's pre-consulted, heavily formatted & non restrictive days) these parody's will be something you will thoroughly enjoy.
I point to both of these url's (and hope Marco won't be upset that I posted them, since they don't have reciprocal links back to this site) but since the comment was posted about listeners tuned into a frequency, and this *IS* afterall a message site about radio, I felt it was totally appropriate to post these links using the example of what =CREATIVE RADIO= when properly produced, with a little wry twist to boot, should sound like.
I believe you will agree, these parodies are timeless and so dead-on when it comes to what radio has become.
Enjoy!
((Radioboy))