Well, I'm not sure when the IOC decision date was first announced, but I had personally heard about it at least month prior to the decision, and I would imagine the date was set in stone even prior to that.
Assuming al Qaida is well established throughout Europe, and I think we have good evidence that it is, there is no reason to believe they could not have had cells ready to go in Paris as well as London (the two top contenders). Perhaps each cell had orders to attack if their city won.
The Olympics is very symbolic, and terrorists love symbolism. The very fact that a city gets the Olympics means that it will likely go down in history as a city of great things. Cities are also chosen that have good public transport, to shuttle all the Olympic goers from venue to venue. These people assume they are safe, until an attack on that very system occurs.
This is a possibility.