The volcano strikes travel plans again! If you have made a specific rental reservation, what you may find is that because many travel plans were extended due to the volcano, some customers have kept their rental car longer without informing the agency. If you can drive a manual transmission and don´t mind taking anything that´s available, you should be OK. But that van you reserved for the entire family, or that BMW 5 series with automatic transmission? It may still be out on the road with another customer when you need it. As you can guess, despite my reservation, I am still car-less for the exact reason mentioned above (grumble, grumble...).
You are less likely to run into this problem at a large international airport like CDG, Frankfurt or Heathrow, where the rental agencies have much larger fleets and more flexibility, than at a smaller regional office like by a train station or a secondary airport.