Since most 3G subscribers today also pay for a data plan (which basically means - Internet traffic), applications such as fring would become more and more popular. Leaving it on for the entire month, just for texting, wouldn't drain even the most basic data plan. Using it for some VoIP calls would push the bandwidth usage and might bring it to the limit. Even so, using SkypeOut or GoogleTalk instead of long distance calls would make the price of a larger data plan quite reasonable.
I believe that eventually everyone would be connected to their IM service(s) while using a mobile device, and would use it for text messages. This would probably mean that the SMS would die, as it wouldn't make sense anymore to pay (even if it is only few cents) for 140 chars anymore.