For some time now I've been wrestling with 3g over here in an attempt to find a PAYG provider which I can use without having to buy a top-up card every month to avoid losing credit. I know it's something which taxes those of you who want an internet connection during the race at an affordable price.
Anyway, although it's somewhat lugubrious for non-residents and doesn't solve the problem altogether, I think I might have cracked it at last!
Leclerc's have now introduced a mobile internet package for 9.90 a month (250Mo) or 13.90 (500). Once the month of June is up then you can move over to a calls only package which charges a monthly fee of 1.50 until your credit runs out, at which time you can dump it. Yes, I know it's a bit long winded but with a SIM-free 3g dongle it should work. Just pop into a Leclerc's supermarket and the staff will sort the thing out for you (don't forget your passport though!)
I'll let you know how I get on!
http://www.leclercmobile.fr/formules/options-sms-internet.html