This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
Without any info on which servlet version the deployment descriptor is targeted, the container can only guess. It will usually guess that you are specifying the latest version that the container supports, but that may not be correct, and probably won't be if you upgrade the container or move the app to a different container.