I just got back from a wonderful 2 1/2-week trip to Provence, the Loire Valley and Paris. Before I left the US, I'd bought a cheap little $30 unlocked phone at Best Buy and intended to sign up with a French network. When I arrived in Nice, I found a phone store with Orange (the big national phone network), showed them my phone, and signed up for a pay-as-you-go plan. The prices were shocking -- 40 Eurocents per minute for local calls and 55 Eurocents per minute to call the US -- but I didn't know what options I had.
A few days and too many phone-dollars later I was in St-Remy-de-Provence. I asked at the TI if they could direct me to a phone store where I wanted to make some inquiries. They told me about this great little shop called Cybertel Saint-Remy where I spent a wonderful half hour talking with the proprietor. It turns out there's a cell phone network called Labara that is much, MUCH cheaper than Orange and is specifically priced for making out-of-country calls, though their domestic prices are also much cheaper than Orange's. Price for calling the US: 20 eurocents for the first minute, then 5 eurocents per minute thereafter. To call domestic numbers in France it's actually more expensive and I think was (I'm doing this from memory) 15 eurocents per minute. I had a great time talking with him. He sold me a Lebara SIM card and signed me up with them, and I was very happy with this service for the rest of the trip. They have a website at lebara.com.