Probably tied to the cost of capital
You've got to admit that Tesla is pretty ballsy - launching a new car in the midst of a depression. And while this pricing "upgrade" may seem like cruel and unusual punishment for their early customers, it most likely reflects the reality of needing to post an early cash flow success in order to be able to go to credit markets for on-going financing. After all, if the banks aren't prepared to lend to GM, Ford or Chrysler (or Toyota, Nissan and Honda for that matter) why should they consider Tesla to be a good risk?
The brilliance in Tesla's move is that it has kept the price "revenue neutral" for its customers. Yes, they "lose" the US$7500 tax credit, but the net-net is pretty much a zero impact. And, as Steve notes in the first post, being able to build and inventory parts is as much a part of the game as selling the car.
Let's hope that Musk keeps his snoot out of the Marching Powder...unlike some other car entrepreneurs we can name...