Here's my experience with 3G PAYG prepaid data in France:
3G prepaid internet access in France with Orange
Info included how to work around the small print to avoid having your prepaid balance zero-ed in no-time. Orange excludes POP/IMAP access from the flat-fee rate, including using the connection on a laptop.
IMHO these small printed conditions are on the sheer edge. A non-tech savvy user doesn't understand these exceptions. It might not even hold in European courts when put on the stand.
Cheers!