You need a blackberry data plan in addition to any voice plan you have.
Also, Alltel DOES NOT provide Blackberry Enterprise Server (BES) with their blackberries. They provide the Blackberry Web Client (BWC or sometimes referred to as BIS).
BWC is an online mail client that either pulls emails from your POP3/IMAP mail server every 15 minutes and then sends the emails to your Blackberry. You can also forward all of your emails to your BWC account for instant email push.
BWC is free, but limited to only 10meg of email space.
All Blackberries with BB OS 4.0 have built-in HTML web browsing without the need for the MDS feature of BES, so if you are looking for HTML web browsing and email features you don't need the extra expense of a third party BES hosting service.
I have a verizon BB 7250.
I have a voice plan for $40/month + bb unlimited plan for $45/month. I didn't see the need for another monthly bill for a third party BES hosting service.