This effectively means that all new customers who expect to send out a reasonable amount of text messages will need to pay an extra $10/month for that service.
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.
Many see this as response to Apple's introduction of iMessage, a new iOS 5 feature that will bypass AT&T's text message charge and send messages freely via your data plan.