The Guardian this morning reports that the Lib Dems are pushing for this referendum to take place in May 2011 at the same time as the Scottish and Welsh elections. There is, as the article notes, a huge benefit to the Lib Dems in getting this referendum in early before the cuts kick in and the coalition becomes unpopular. But I suspect that the Lib Dem leadership also wants the referendum then to soothe the party with if election night sees the Lib Dems punished for going into coalition with the Tories.
One thing to watch is if there is any softening in the Tory position on AV, David Cameron has told Tory MPs that he'll personally campaign against it. Already there are people in Westminster murmuring that AV wouldn't necessarily be bad for the Tories now. The argument goes that the coalition means that the Tories have a far better chance than before of picking up second preference support from Lib Dem voters.