What model BlackBerry do you have?
Cingular has an interactive web site -
http://onlinestorez.cingular.com/int...guide/wtg.html - where you can check their roaming agreements. Check out the roaming costs from the Cingular web site.
Activating your phone for international service should not cost you anything. They offer some sort of unlimited international plan, but that is not necessary for the casual traveler and is not required.