I emailed Showtime the other day and here is the response I got from them:
Thank you for contacting us. After some investigation with our engineers, we have confirmed that DIRECTV is broadcasting both Showcase (Showtime 3) HD and Showtime Extreme HD, with around 60% TRUE HD programming (virtually all TRUE HD in primetime). Can you please verify that you are indeed watching the High Definition channels and that your receiver is correctly set to receive HD programming? You may also want to try contacting DIRECTV if this technical difficulty persists.
Thank you again for contacting us, and thanks as always for watching Showtime.
Showtime Customer Service
This seems to confirm what others and I have noticed. The HD is there, just not listed that way.