It might also have something to do with Sky+ HD box manufacturing, in terms of them not having any...
As with it's standard Sky+ box, Sky has outsourced the the manufacturing contract to numerous companies including Thomson, Samsung, Amstrad and Pace. The idea being that the units are then used and "field tested". The highest rated from user experience test groups and/or the most reliable unit's manufacturer is then given exclusivity in manufacturing the unit.
If this is the case, it may be that this deal has now been made and might account for a temporary shortfall in available stock whilst the transition completes.
Complete conjecture of course, but another possibility. It might be in your favour if you end up getting a reliable unit!