Cheater, this is an interesting problem. The answer depends on what your future intentions are with your original boyfriend. Now, you said that you aren't into the idea of an "us" which leads me to believe that the two of you have already talked about or seperated. Basically, it sounds like the exclusive relationship between the two of you was ended.

So, now comes the advice. If you have any intention of dating your original boyfriend, either just for fun or as something more serious, then you should definately tell him that you cheated. Since this is something that occured during the original relationship, he deserves the right to know all about the things that you've done before he agrees to get back together with you.

Now, if you're planning on just totally ending everything between the two of you, I would say that you don't have to tell him about your cheating. If it's something that you feel very strongly about, then go ahead and tell him. But, if you are planning to end it all between the two of you than I'd say to not tell him.