Even if the Yes camp wins the electoral reform referendum, there's no guarantee that the Alternative Vote will be used at the next election. As the Electoral Commission's guide  to the referendum points out, it depends on the successful completion of the boundary review.
The booklet notes:
The "alternative vote" system will be used after a review of the boundaries of the area that each MP represents (known as their constituency) is completed. This is due to happen between 2011 and 2013. The review will happen regardless of the outcome of this referendum.
At the end of the review, the UK parliament will vote on implementing the new boundaries. If the new boundaries are implemented, the "alternative vote" system will be used for all future elections to the House of Commons.
In other words, if, for whatever reason, the boundary review is not approved by parliament in time for the next election, the Alternative Vote will not be used.
The Electoral Commission press office confirmed that if there's an election between now and 2013, it will be fought under first-past-the-post.
Among other things, this provides the Lib Dems with a clear incentive to remain in the coalition until 2015.