If you have a mobile phone that is tri- or quad- band, you should be able to add a SIMM card when you get here. The deal is that phones over here operate on different bands than those in North America. Many of the Japanese brand phones are multi-band, so if you already have one of those, you're all set.

I would tend to stay away from the phones for sale at the airports. There are tons of phone shops all over France - some are run by the mobile phone operators (Orange, Bouygues or SFR) while the big hypermarkets and electronic gadget stores all have mobile phone sections where you can compare the offers across a variety of plans.
Cheers,
Bev