I.M. via GPRS

Is there an application that allows Instant messaging via GPRS. I have service with AT&T wireless and I use about 600 text messages a month, and there highest pacakage includes 300. I end up paying $30 a month just for text messaging. I have there unlimited pacakage for internet service so I was wondering if there is an Instant Messaging Application that I can install on mine and my wifes phone that will run off of GPRS instead of Text messages. I have been searching and found a couple of apps but I am not sure if they use GPRS or (SMS) text messages.