Confidence trick?

Then:

‘Ayes, 311. Noes, 310.’ Even before the figures were announced by the tellers, we on the Opposition benches knew that Jim Callaghan’s Labour Government had lost its motion of confidence and would have to call a general election. … A great burst of cheering and laughter rose from the Tory benches, and our supporters in the spectators’ galleries roared with out-of-order enthusiasm.
– Margaret Thatcher, ‘The Downing Street Years’

Now:

MPs will not be able to throw out the government unless at least 55% of them vote to do so, under plans agreed by the Conservatives and Lib Dems. The move would protect David Cameron from losing power even if the coalition partners decided to split up.
…
The Conservatives currently have 306 out of 649 MPs - a 47% share.
…
All this means it would be impossible for opponents, even if fully united, to muster the 55% needed to oust the government

Update: According to Left Foot Forward, the BBC (and other media) may have over-interpreted this and confused a confidence motion with a dissolution resolution. Either way, the concept of a majority being defeated by a minority is a new one for Westminster.

And this does feel rather as if the constitution is being changed for the convenience of one particular government: this rule would mean the Lib Dems couldn't walk out and support a Labour-dominated government supported by the other parties, and the Tories couldn't just call a snap election at will if they thought they could get a better result. It stops either party ratting on the other, which I'm sure suits them in their agreement, but I'm not convinced that this is what legislation is supposed to be for.

Update 2: I've just seen Chris Rennard, a Lib Dem peer, explain the rationale for this on Newsnight. He was stunningly, shamelessly open about the fact that the government is buggering around with the constitution because it suits the two parties, as otherwise they'd not trust one another.