Cell phone in France

I am carrying an original iPhone, and have it in airplane mode (i.e., was not intending to use it, as is, for phone calls in Europe). I will be getting a new iPhone when back home (after the new models are out). When I get to Paris can I just pop out the SIM card from my old iPhone and purchase one of the ones as you describe here? Does the "breaking" of the phone involve more than popping the SIM card?

Thanks to everyone for their helpful responses. I did go out and buy a used GSM-compatible phone (a Motorola) and understand all I have to do when arriving at CDG airport is buy a pre-paid Sim card. But here's the followup question: does it matter which brand of Sim card? I've been told that the Sim card is basically the carrier and that the phone number that will be transferred to the Motorola is embedded in the Sim card. So does it matter which carrier in France I use? Thanks.

It doesn't matter, but you might shop for good rates before you buy.

We buy a chip for our cell phone when we get to France, but we only use it for emergencies, etc. If we need to communicate with children and family, we find an internet cafe, buy 15 minutes of computer usage for a euro or two, and compose an email and send it to everyone. In a day or two, we check emails again, and answer specific questions or just tell everyone we are ok and are moving onward to . . . That way, we do not have to worry about being 6 hours ahead and waking someone up, etc. Email will wait till they check it.

You can buy a GSM sim at the Airport and it shouldn't matter as long as your phone is GSM chip compatible. Stateside that's ATT or TMobile. In addition I recommend calling your bank and telling them to put an internation travel alert on your bank cards(so they don't throw a fraud alert on them and you can't use it). Or you may spend a lot of time on the cell phone.