Cameron scored a direct hit at PMQs because he knows Brown is isolated on the Additional Vote system. You only had to observe the reaction on the Labour front bench when Cameron quoted Paddy Ashdown. Also, he would have been told what Eric Pickles picked up last night.
As Iain Martin correctly points out:
Today the Tory leader will have boosted the morale on his side, but it’s only PMQs with a small audience and their wider problems remain.
However, putting Cameron's problems to one side, if Hoon and Hewitt had published that e-mail today, the outcome may have been very different.