It seems that the right strategy for AT&T to pursue would be to follow the natural decline in the cost of these services by offering a better plan-- say $5 for 2,000 messages a month. As they lower the cost of an individual SMS, the usage should go up dramatically, and since SMS has network effects, this might slow the shift away from SMS as the messaging service of choice.
Assuming your conversation partners can keep tabs on what system to use. Your Android pals can't iMessage you, and using Google Talk or another IM service on an iPhone has always felt a little kludgy.
Sounds to me it's designed to drive you to one of two choices: either pay $20/month for a service you don't need, or cut it out altogether and pay 20c every time you get an SMS from someone who can't use iMessage/Twitter/Facebook messaging.
At 20c/message it doesn't take long before you end up paying as much or more than you would've been paying under an SMS plan.
It would be a great move by Apple to open the iMessage protocol so Android and other smart phone OS' could participate in the exchange. Unfortunately, my guess is that this won't happen as "free texts" represent a great benefit for switching to an iPhone and opening that network to other vendors would undercut their advantage.
Predication: ATT/Verizon drastically increases the cost of bandwidth on iPhone plans to make up for the cut. We'll see...