I have verizon, and they do require you to purchase the Blackberry Unlimited (sorta) plan. Which is an additional 29.99 a month if you are not on a corporate server.
I never seem to get a staight answer from the people that I talk to on the phone, I don't think they have alot of experience with blackberries so they sort of fumble their way through an answer. I'm sure they know all their is to know about text messaging and all the normal phones but whenever i have a billing question about changing plans involving the blackberry part, they always seem to be clueless. The tech support that I have received personally for the blackberry has been top notch (so far) so that is a plus.
Good luck in your search.