Dirty, sounds my like ex too...who I was with for 11 years! But despite what we went through in those relationships, and the years we lost, we're better women because of them.
Anyhow, as far as telling him....that's a tough one. I would say, don't tell him....that's not to say you should keep it from him. If one day he asks, you tell the truth...that your ex contacted you and you took care of it. You handled it. I just don't think all things need to be said when it comes to your past. Your ex is part of YOUR past. If YOU contacted him then I could see why that is something you should tell him, but you didn't. If he's the jealous type...don't tell him. Some men can't handle it...and he'll start wondering all the time now who else is contacting you on myspace or if your ex is going to contact you again blah blah.
However, if you're dead set on telling him, then say it in a very non-chalant way. If you make it a big deal, he may think there's feelings still there. I don't know. That's just my opinion. Men are funny beings...and even though all we women here think your response was AWESOME, he may think that your message was full of a lot of emotion, even if it was negative emotion....do you know what I mean?
Good luck and let us know how it turns out!