As expected, this has been scheduled to coincide with the local and devolved elections next May in the hope of boosting turnout. One of the problems with PR as an issue is that although it excites the political class it doesn't feature in the most important isues selected by the electorate in the polls.
In many ways it will be a watershed for the coalition. If the Liberal Democrats win the referendum, then they will have got one of their most sought after lasting gains from the Coalition - although in practice AV is just a modified version of first-past-the post and isn't going to make a massive difference to outcomes. It certainly may not offset the loss of left-leaning votes the Lib Dems are likely to suffer. But, of course, it could be the start of a slippery PR slope.
If the referendum fails, which is the less likely outcome, then Liberal Democrats will be asking what they have got out of the coalition given the sacrifices they have had to make on other policies. Either way, tensions within the coalition will increase at the time that public expenditure cuts start to bite. Already the Lib Dem MP for Cheltenham is complaining about the closure of the Commission for Rural Communities quango in his constitency with the loss of 60 jobs.
My guess is that the Coalition Government will not be able to eliminate the structural deficit in the lifetime of the Parliament because the political costs will be too high. It's an ambitious target, but certainly preferable to Labour's unambitious one which would not have reassured the financial markets. However, I think there may be some trimming as the electoral cycle progresss and if the strictural deficit is reduced by, say, three-quarters, that will be a result.