here). The rationale for this seems to come from a desire by the parties forming the coalition to lock themselves into a fixed term arrangement for government. A fixed term parliament would take away the Prime Minister's nuclear option of controlling the election date but, even if one accepts that fixed term parliaments are a good idea, there ought still to be a debate about whether the term should be 5 years or 4. Most parliaments seem to be running out of steam at around 4 years.
The Coalition's proposal goes on to say that the legislation will "also provide for dissolution if 55% or more of the House votes in favour". This part of the proposal certainly requires a great deal more thought and explanation. For example, in view of the coalition arrangement, is the proposal limited to this parliament or is it intended to be a permanent change? On the face of it, there could be many possible problems and perhaps not all of them can be identified at the present time. Even if the parties to the coalition government wish to lock into a 55% arrangement (should they wish to propose a dissolution) it would seem wise to retain the traditional "no confidence rule" - i.e. that if the government loses a vote of confidence by at least 50% + 1 then the P.M. must resign and thereafter a general election is called.
For further analysis on this see the blogs Head of Legal and Charon QC.
Addendum 14th May 2010: See Daily Mail - "Revolt looms over 5 year parliament "stitch-up" that Liberal Democrats demanded" and also see BBC 14th May.
Further addendum 14th May 2010: - Now that the possible implications of the 55% proposal are being realised, many politicians are speaking out against it. The Times 14th May.