We've primarily had this problem between Ireland and Italy. Even though there is a GPRS roaming agreement between the carriers, users roaming don't get more than a lower case gprs - and this happens no matter which carrier you select.
If you are sure that there is a GPRS roaming agreement, then there's one fix that we've used in the past that might work. I'm not 100% sure why it works (perhaps someone here can explain it!
) but it's part of the list of things we now try if we have a problem with a roaming user.
Firstly, you'll need a mobile phone that supports GPRS.
Take the SIM out of the BB, put it into the Mobile phone and activate the GPRS service (doesn't matter what you do - just go to one of the generic bookmarks).
Put the SIM back into the Blackberry and do a reset.
Turn wireless back on and do a manual network select.
I don't know if this is applicable to your problem, but it's worth a try!