I'm confirming what Dan said. Blue text messaging bubbles means you are utilizing Apple's new iOS 5 feature, iMessaing, which means you are texting another person using an Apple mobile device with iOS 5 such as an iPad, iPod Touch, or
iPhone. These text messages are real and do not count toward your carrier bill. Green text messaging bubbles mean you are texting someone who is using a non apple device or they simply haven't upgraded to the latest iOS mobile software.