In answer to the many questions about prepaid 3G data for an iPad in France, I have just returned from a trip to France with a 3G iPad and the situation is actually very positive.
Use the SFR website (www.sfr.fr) to find your closest SFR shop (most towns have one) or Google for 'espace SFR [name of town]'.
Print off the following page from the SFR website before you travel (sfr.fr/mobile/…) which shows details of the prepaid iPad 3G data micro SIM.
All you need to do then is show the page to the sales person and point to what you need. Even if you speak French, it's a good idea to have proof that what you want actually exists!
What you want to buy is the 'Kit pret a surfer iPad' ('iPad ready to surf kit'). You will need photographic ID (which they will photocopy in the shop) and a local address (your French hotel address will be fine).
The SIM costs Euros 14.99 (at August 2011) and includes 3 days of 3G data. Ask the sales person to help you install the SIM and check that your data link is working. If you don't speak French, just pantomime what you need.
My SIM fired up immediately and automatically picked up the APN (this is the name of the network (for SFR it's: websfr) if you ever need to add it manually go to Settings, Cellular Data, APN Settings and enter it in the APN box. There is no username or password).
Top-ups are available on a time basis (i.e. per day) or on a volume of data basis (50Mbb 100Mb, 200Mb, 500Mb, 1Gb) Examples of the top-ups are shown at the bottom of the web page mentioned above and I strongly suggest that you buy what you need at the same time as buying your SIM.
You can, in theory, top up online via the SFR website but only with a French credit/debit card, hence my advice to buy your credits in advance at the SFR shop!
Top-ups are also available at newspaper shops (La Presse) and supermarkets. The top-up is just a paper receipt with a 10-digit number that you type in to your iPad.
I have documented my experiences with an iPad in France on my blog (http://blog.utting.org).
You can see how much prepaid credit is left on your iPad by going here: https:/…026_loginInfoConso.do
You can see how to add fresh prepaid credit to your iPad here: …utting.org/2011/08/06/ipad-in-france--the-f…
SFR's 3G service is widely available and, in conjunction with (often free) hotel and cafe wifi, there is no reason to be offline in France. In cafes, the wifi security code may be printed on your receipt, or just as for 'Le code pour le wifi' (pronounced 'weefee').
Happy prepaid surfing!