The holdup is clearly the price negotiations with the channels.
I thought it was very likely we would see new Viacom HD channels, since all the top rated channels not on Dish in HD are from Viacom.
However, Eric Sahl said on the Charlie Chat (only 10 days ago), that he was still in negotiations with Viacom.
So, I think that as of today, Dish Network has plenty of satellite space for new HD channels, but no contracts to add those channels to their packages.
PS The MPEG2-HD to MPEG4-HD conversion will happen between midnight ET and 4am ET - not during the day when it would disrupt viewers. Again, the holdup is not satellite space, but rather waiting for installers to finish swapping 811s, 921s and 942s for 211s, and 722s. It is more important to E* that customers not lose their HD channels that they subscribe to.