AT&T has confirmed to Engadget that, as of August 21, they'll be removing all but the unlimited SMS/Text messaging plans:
"We regularly evaluate our offers and are making some adjustments to our messaging lineup.
Starting August 21, we're streamlining our text messaging plans for new customers and will offer an unlimited plan for individuals for $20 per month and an unlimited plan for families of up to five lines for $30 per month. The vast majority of our messaging customers prefer unlimited plans and with text messaging growth stronger than ever, that number continues to climb among new customers. Existing customers don't have to change any messaging plan they have today, even when changing handsets."
With iOS 5 and iMessage likely set for an early October launch, AT&T was facing a world where Apple provided an easy, integrated way to seamlessly exchange free instant messages inside the same Messages app that hosts costly, carrier-based SMS/text messages. Arguably, a subscriber who's friends and family were also mostly on iPhones could have downgraded to a lower plan to save some money.
Now that can't happen. It's unlimited or nothing. And as MG Siegler points out on Paris Lemon, it's quite a coincidence.
SMS is huge money for carriers considering the data uses is next to nothing and the price is stratospheric, and iMessage was a threat to that traditional, exorbitant revenue stream.
Now we're just left to wonder which other carriers will follow suit.