Strange that a breakdown isn't being reported. But 20,000 units sounds pretty insignificant to me. Does that consider sales of the full collection as well? Amazon's sales rankings of the Mission: Impossible Collection place the HD DVD edition at 423 while the Blu-ray edition lags far behind at 2883.
The interesting numbers will be when Fox releases X3, and Universl releases King Kong (excluding numbers from the free giveaway). At that point, with Mega movies being released in both formats as exclusives, and MI 3 being a cross platform film, the exclusive studios can begin seeing what exclusivity is costing them in lost revenues.