Green Dot in eMail

I received an email from a friend and there was a green dot to the left of their name in the email. Curious, I contacted Apple and found out that it means that person is available for Messages. That was included in the last Mt. Lion update. 

It would be nice if Apple would tell you what the latest updates include. 

BTW, as soon as my friend closed his computer the green dot in my email went off.