There was a thread in which (maybe) BRAVO or some channel mentioned that Dish has 3 or 6 million HD households (I'm thinking it was the 3 number). Given that Dish has 13+ million total, well, someone's speculation was rather inaccurate.... SD customers still far out-number HD customers by a long shot.
Who knows what was misquoted. Could be the number of legacy receivers or those incapable of receiving an MPEG-4 signal. However, I think there are still a lot of legacy receivers out there, more than this statistic would indicate.