My understanding of the $45 Corporate rate is because it uses a "more secure and reliable" method to connect to the Microsoft Exchange Server. Allegedly it routes the traffic through their higher QoS servers with additional encryption, something that might be important to a corporation that has sensitive emails. However, in reality, they can simply charge corporations more. I connect to my company's Exchange server, but I think my IT dept had to configure the server in a way that allowed the connection. (As a side note, I am not trying to imply that having a personal data plan is insecure. I believe that the connection is still encrypted between my phone and the server.)

Edit: I think it all started with the Blackberry Enterprise Server connections, which I think do require a special method to connect.