Theology Thusday: Does the Old Testament Teach the Trinity

A few months ago, I got an E-mail from a friend.  He quoted Genesis 1:26 and asked me who is the "us" mentioned.  That got me talking to him and a few other people about the Trinity and the Old Testament.  Does the Old Testament teach the Trinity?  If not, then why not?  If God really is one Being who eternally existed as three Persons, then why would He not feel like mentioning that in the Old Testament?