There's an app for iPhone and iPod touch called "Ping!" that lets you send messages directly between iPhone/iPod users, without using your SMS message allotment and without having to be "signed in" to an IM service. It uses the push notification feature to enable your iPhone or iPod touch to notify you, whenever it's connected to the Internet, of new incoming messages. There's no charge for sending or receiving messages; you're just exchanging messages using your Internet connection, whether it's via Wi-Fi or a cellular data connection.
For a limited time, the app is free. (Once it's installed, you can purchase the ability to send pictures, as well.)