If you are under iOS 5 send a text message to another iPhone user iOS 5 iMessage will try to send message via data link. This is obviously both iMessage iPhones to be activated.
If on the iPhone where you want to send the text message data connection not available then iMessage after a timeout message when sending a normal SMS message.
If you know the recipient does not have access to a data connection (eg stay abroad), you can force a text message to be sent directly to a text message.
Type your message and tap on the button 'Send'. Tap directly below the balloon and keep your finger a few moments on screen. Will automatically pop-up menu appears that allows you to transmit directly as 'Text Message' (SMS) can be sent.