The funny thing is that from his speech, all Cameron had to do was accede to a referendum on voting reform and I reckon he could be in No10 already.
Would it really be so hard to say "Ok we'll agree to a referendum, but will campaign for FPTP"? Or does he realise that such a campaign might ostracise the Tories from the general public?
I think he is more worried about his own party turning on him. Your average tory would rather lick gordon brown than have a referendum on PR!