In a recent interview Cameron defended the indefensible first-past-the-post system, but he left open the door that he might be willing to have a referendum on changing the system.
He did this for tactical reasons. Firstly, he wants to be PM and if a deal on PR with the Lib Dems is the way to do it, then so be it. The Tories would only do this however if they knew that a Lib-Lab deal on PR was on the cards. Secondly, this is good electioneering - trying to soothe any floating voters who are undecided between Lib or Tory.
Also the Tories know that if they are in charge for the referendum it would make it easier to influence the outcome and crucially which system is offered. I imagine the Libs and Tories can come to a deal over a 3 (or even 2) seat STV system. This would set an extremely high threshold for smaller parties to pass of over 25%. A Labour/Lib Dem deal on PR might lead to AV+, AMS or even a 6 seat STV being offered, as Labour would insist on a more proportional system with a lower threshold to allow them to win seats in rural areas.
Cameron would do everything he could to delay a referendum and double cross the Lib Dems - calling an election early if he could to avoid PR. Labour tribalists might also attempt this, but Labour at least does have some high profile advocates of PR amongst its ranks.
It is still possible the Tories are going to get the seats for a majority (especially nor Brown's gaffe has happened and the Con-Lab gap could grow), so lets not get too excited but finally people are beginning to see just how corrupt our system is. Even the general public uninterested in constitutional details can see that something is seriously wrong when a party can win the vote but come last in terms of seats.