AT&T no longer forcing messaging plans onto texting phones

"It was a flat-out confusing requirement," noted Matthew Kunkle, an analyst at Current Analysis. "It was a $20 messaging or data package requirement, yet most of the data and messaging packages weren't $20, so customers had to combine two to make it past the $20 level."

AT&T now recommends that purchasers of its messaging devices also purchase an unlimited texting plan, but it does not require such plans.

AT&T no longer forcing messaging plans onto texting phones - FierceWireless

They made some changes to their plans in Jan, 2011.
One of which was dumping the $5 texting plan for the minimum $10 Messaging 1000 plan

Here is their plans as they are now:
Get Started - Wireless from AT&T