The idea of a fixed parliament is a reasonable one (although the 55% rule takes away from the proposal unnecessarily). However, the implementation of the policy and if successful, the other Con Dem proposals could create some localised difficulty.
The next Westminster election would fall on the day of the English local government elections in 2015. In Northern Ireland it would also coincide with an Assembly election and local government elections (to the 11 council model if present proposals are implemented). Then you have to remember that the Coalition proposes a significant reduction in the number of MPs. If implemented this would mean the loss of 2-3 constituencies, it is hard to see how any constituency wouldn't end up with significant boundary changes. If the AV referendum passes then a new voting system will have to be implemented.
So it is possible we will have three elections on one day with potentially two of them on new boundaries and one implementing a new voting system. The debacle in Scottish elections provide something of a warning of how big a mess could arise. If this scenario does occur or something comparable then delaying the local government elections to 2016 may be necessary.