fvp:

Ping! is iPhone-to-iPhone messaging that also works with the iPod Touch. Ping! works with push notifications so that messages can be delivered ANYWHERE in the world, instantly.  The experience is simple and familiar (resembles SMS!) but does not incur additional carrier charges, like text messaging would.

Ping! is similar to BBM in that you can message other iPhones and iPod Touches quickly and easily without text messaging costs. Ping! is intended for your friends and works only when you have your friend’s Ping! ID to start messaging.