My fear is that this is seen as essentially the LAST thorough firmware release and the reason they are taking so long to release it is that they don't want any mopping up later -- they want to be done with it so they can move on to important things.

As I say, this is just a fear of mine, and I have no evidence it is true. But the desire to finish off the N900 and not leave anything left to do would explain the delay.