This question continues to bother me and I've spent a good portion of the afternoon trying to find the answer. It is not readily available, but I can make some educated guesses:
While approximately 9% of the US television households have an HD monitor, only approximately 2% actually subscribe to HD content. There are approximately 100 million TV households in the US, meaning roughly 2 million subscribe to HD content. Even if all of these people received their HD content from DirecTV, this is the MOST it could be (2 million).
If you apply that same 2% penetration rate to DirecTV's 12.6 million subscribers, that would mean about 250,000 receive HD content. Given that the satellite companies until recently had the lead in HD content over cable companies, I can assume that the HD penetration rate is higher among DirecTV subscribers than overall, so 250,000 is the LEAST it could be.
My best educated guess is that about 500,000 DirecTV subscribers currently receive HD content. That would be twice the overall penetration rate, and suggest that DirecTV has 25% share of the approximately 2 million US television homes subscribing to HD content. Both of those seem logical and pass the smell test, so I'll stick with my estimate of 500,000 unless someone can prove me wrong!