In the coalition agreement that Cameron has persuaded Clegg to sign up to, they propose to change the constitutional provision for removing the Government and forcing a General Election. Hitherto, this has required only a simple majority in a 'No Confidence' motion voted on by Parliament. Cameron wants to change this to require the vote to be by a majority of at least 55% of MPs. Now then, even if the Lib Dems get fed up of the coalition and vote against Cameron together with all the other MPs that only adds up to 52.8% of the total. So to unseat Cameron, there'd have to be some suicidal Tory MPs joining the opposition, willing to risk losing their own seats in an election.
Cameron intends to introduce this change by a simple motion in the House of Commons. In addition, he intends to introduce a fixed Parliamentary term of five years. So, in effect, he's guaranteeing himself at least five years in the job. Plenty of time to set yourself up for a lucrative post-Parliamentary career.
I remember the Alternative Vote system being discussed in the election campaign - it was in the Labour Manifesto. But I don't recall anything about these particular proposals. Does he think we haven't noticed?