One of the main constitutional innovations the new coalition government wishes to bring in is a fixed term 5 year Parliament. Apparently the Lib Dems were especially keen to have this, as they feared the Conservatives would use their support until the polls suggested the Conservatives could win an overall majority and then go to the country at a time of their choosing. Of course any looser agreement between the two parties also left the Lib Dems free to dictate the date of the next election as well, as they could withdraw their support at any time.
There are two main reasons why we have not in the past had fixed term Parliaments. The first is the Leader of the governing party has valued the ability to decide when to hold the election in the political interest of his party, and majority parties have usually thought that a good idea. A coalition obviously takes a different view. The second is that binding a Parliament to five years does not allow for accidents which could fail to produce a majority for any government. The right to hold an earlier election could be needed if, for example, a government with a small majority loses that majority through deaths and defections, or it could be needed if the governing party split apart on a big issue.
In practise, the Parliament which enacts a Fixed Term Parliament Act could also repeal such an Act. So if the Lib Dems did wish to end the coalition early and had the support of Labour and others for doing so, I guess they would just move the repeal of the Fixed Term Parliament Act. It is a lock on the door, and finding the key would be difficult, but not impossible. There is some talk of weighted majorities to repeal, to try to make such a course even more difficult. We will have to see whether these are either technically feasible or desirable.
It is a fairly safe assumption that we have a five year coalition government stretching ahead. Senior Lib Dems have decided to surrender some independence of thought for the opportunity to influence a government, and may well like the experience. The UK does need stable and strong government. Having a majority of around 70 and saying you will legislate for a 5 year Parliament gives you as strong a platform as you can create in the cirucmstances. What matters now is what they do – and events.