I'd also ordered a free Lebara SIM and received it today. While it's supposed to be mailed within France only, I'd read of others' success in just putting USA by the state.

The SIM was already activated and had the phone number printed on an accompanying sheet. I cut it to micro-SIM size and inserted into my old unlocked iPhone 4 and it picked up an AT&T signal.

I then went through the process on Lebara's site to setup an account, used my credit card to top-up €10; all worked just fine, and my phone was showing the correct French number. I dialed this French number from another phone and it works just fine.

This is a voice-only SIM and actually has reasonable rates to call back to the US while in France (cheaper than calling a French number actually); €0.05/min plus a €0.15 connection fee. This will be a good way for me to call back home during the trip.

Pretty easy and straight-forward process, and I now have at least one known French number available to use as soon as I land.