You can log on to your AT&T account. How many you have used is listed on the data tab. Be warned however that unlike minutes, these are not "up to the minute". There can be a day or two delay in posting them.
This is very easy to do with the iPhone. To check text messages dial *3282# and you will be sent a text message with how many text messages you have left (or how many you are over). I have this saved as a contact in my favorites list.
and i think i'll go home and watch Billy Madison tonight..
actually, i'll(not being anyone significant to anyone on here) make it a mandatory event that EVERYONE on the boards has to watch it tonight..