Before plunking down 650 euros (about US$965) for an "unlocked" French iPhone, take note: the device retains a country lock meaning that it will only allow use of SIM cards for carriers that operate in France according to our friends at iPhone Atlas. In other words, you can't take your France-bought iPhone to Spain, the United States, or anywhere else, pop in a foreign SIM card, and make calls on a local carrier; you're still stuck paying international roaming fees to a French company.
Attempting to use a foreign SIM card with one of these partially-unlocked phones will result in an inability to make calls, send text messages or use other network functions.
This is almost certainly a restriction imposed by Apple, loathe to see "unlocked" phones shipped from France to the US or other countries where exclusive contracts are in place with local carriers.
France is the only country where you can purchase an "unlocked" iPhone. In the US, UK and Germany (the only other markets where the iPhone is available), Apple holds exclusive contracts with AT&T, O2 and T-Mobile respectively. French law forbids the sale of one item that requires the purchase of another.