Having recently spent some time in France, one of the first things I did was attempt to acquire a 3G data SIM for my phone. Having a local sim makes it really easy to get around using various smartphone apps (eg maps, train timetables and general internet goodness).
After a fair bit of research, I realised that there are no official mobile carrier outlets in the airport. So there is no easy option. Buy you can get a limited access sim from a newsagent..
I got to Charles de Gaulle airport in Paris, and discovered that the SIMs can be purchased at the "Relay" newsagent stores there. I got mine from the outlet in the car rental hall. There is also an outlet near the arrivals gate at 2D, though I don't know if they also have SIM cards.