I think the same supply issue occurred when the HR21 first came out as well. Directv should know by now how to handle this sort of supply issue. Either crank out more units, make no promises on which HDDVR will be supplied (I don't think any such promise was made with the OP's order), or push out install dates so that there is not a crazy mess that installers have to deal with.
The problem is, and it's not just DirecTV, that to introduce a new model you must stop making the old model. You can't continue to make HR23's and start making HR24's at the same time. You can't make the 2010 Camaro at the same time that you make the 2011 Camaro and so on.
The only way to really effectively handle the situation is to stop doing installs when you quit making one model and don't start doing installs again until you have a sufficient supply of new models. Unfortunately this won't work in the world of business.