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.