Yes they do. We use SFR onboard or the Post Office ashore.
Be warned - the Post Office only use Apples Macs and there are plenty of sites that you will not be able to access - inclding in our case our Internet bank.
I found this site helpful for the GPRS details for each network: <A target="_blank" HREF=http://www.access-sys-eu.com/GPRS_Settings.95.0.html#402>http://www.access-sys-eu.com/GPRS_Settings.95.0.html#402</A>.