I just want to use my blackberry for voice and data - how do i disable data roaming?

Open up to your Applications menu and scroll to Manage Connections. Underneath the check boxes you’ll see a selection which reads Mobile Network Options. Scroll down and click the trackball on it.

There you’ll be able to see available mobile networks along with a few options, including Data Services. You can turn it off, or turn it off only when roaming. That’s quite a useful feature, as it can save you when you don’t know you’re roaming.