Will a French SIM card work in Belgium?
 

We're going to buy a SIM card when we arrive in France this May. Will it still work when we go to Bruges and Brussels? (Belgium seems so close, almost France, being partly francophone and all.)

It will almost certainly work. What I do not know is how much the calls will cost. The price will depend on the service conditions of the SIM card.

It´ll work, but you will pay roaming fees.

It will work for sure but in France all incoming calls will be free and out of France you going to pay roaming fee to receive a phone call as grrr said. You might also pay more to make a phone call out of France with a French SIM card.

Janicka, I don't understand your last sentence:

< You might also pay more to make a phone call out of France with a French SIM card. >

Do you mean I'll need a special SIM for both Belgium and France, and it will be more expensive for calling in France. Or did you mean to say a call out in Belgium might cost more using a French SIM card.

SIM card is normally cheapest to use in the country where it is bought. So a French SIM card, while it will work in Belgium, will cost more. The EU has set the maximum charge a network can impose when roaming outside its territory, which you can see in http://en.wikipedia.org/wiki/Regulat...European_Union
There has been a decreasing scale of charges each year since 2007, and currently you pay a maximum of 0.39 euro/min (plus VAT) for outgoing calls to anywhere in EU, and 0.15 euro/min for any incoming calls. Some networks charge less than the maximum.
So to answer your question, yes, your French SIM will work in Belgium and, while you pay slightly more, the charges are capped and reasonable. For a typical traveller who visits several countries in Europe, there is no longer any point in buying separate SIM in each country, taking into account the cost of buying and the inconvenience of changed phone number, esp for people trying to call you.
One caveat is that any calls for destinations outside of EU or Europe isn't capped, and you may pay considerably more calling US in Belgium than in France. My advice is to keep such calls as short as possible, and use other methods for outgoing calls, such as Skype and calling cards.