I'm paying $44.99 per month for Xpress Mail with Blackberry Unlimited. I also pay for a voice plan on top of that. My modem connection is working on that plan.
What Cingular once called Data Connect now seems to be called Laptop Connect. An unlimited plan on it is $79.99 per month. When I have read "Data Connect" on this thread I thought it referred to Laptop Connect.
I believe you do have to pay a for the $44.99 ($49.99 if you don't have a voice plan) service in order to do a TCP/IP (modem or Verichat) connection.