There is only one data plan for the Centro, PDA Personal, which costs $30/mo. (The webpage says $35, but that's if you don't have a voice plan.) You could use your phone is pay-per-use (basically, you just don't add a data plan) but you pay $0.01/kb. At that rate, $30 of pay-per-use gets you 3MB. That's not very many messages, depending on the size of the message. If your friend sends you an attachment, then it could put you over the $30 in just one or two messages.