AFAIK - When you sign up for any service with a Blackberry, you are required to get the BB data plan. You don't need to keep it, but you are usually required to sign up for it. This may be company dependant, so not every provider may do this. I know that AT&T does, but like I said, you do not need to keep it.
Text messaging is done seperately, and does not use the internet. MMS (multi-media messaging or picture messaging) does use the internet, and if you do not have a data or internet plan, you will be charged the going $/kb rate.