According to the BBC Olympic vote blog, voting has started. There may be multiple votes to get it down to the city with 50% or more of the votes.

It's down to London vs. Paris at 12:52! Didn't we all know it would end up this way? Who's surprised that Moscow went down for the count? They could have cut out everyone but London and Paris months ago...

It would have saved some money for the cities that were eliminated. And overall it does seem like it was a waste of a lot of money for some places. Then again, you never know until you try....