You can get access for $6.95 to a study of the HHonors program published in the Harvard Business Review in the late 1990's. Admittedly this is old data, but the numbers reported (in much fuller detail) there were diamonds, just under 1% at .9%, gold 8.3%, silver 26.2%, and blue 64.6%.

The only thing is that was with only 2.5 milllion reported members in 1998.
My guess is that with all the promotions the number of golds is now quite a bit higher but that diamonds has not budged too much. Obviously as other posters have reported that HHonors now has 25 million members, the numbers may not be reliable but they do give you a clue.

http://harvardbusinessonline.hbsp.ha...OISW?id=501010

btw, this report has become a staple as a case study for college marketing programs around the country.