I travel a lot (11 countries this year) and brought my BB to all of them. If you have World Class service (international roaming) then you'll be charged by the KB ($0.015/kb or $15 a MB). If you have background apps that autoupdate, then they'll be running up the bill.
For my 8320, I set it so when roaming, data services are off. I figure that if I can find a free wifi spot, I'll turn wifi on and sync data in order to save myself some cash.
But I've noticed that it varies. Sometimes I don't get charged for data when I'm on GRPS (I'll almost always get charged when on EDGE). All I can say is that you need to watch your phone when going overseas otherwise you might find an expensive surprise upon your return.