My sister-in-law goes to a church in Seattle called Mars Hill. Their pastor, Mark Driscoll, is becoming a nationally recognized Evangelical speaker. This is a sermon he gave on the doctrine of the Trinity. He is typically not comfortable preaching unless he can talk for a full hour. This gives him the ability to hit just about every issue related to the Trinity in this sermon. If you have any questions about the doctrine it is most likely answered in this sermon. It’s really a comprehensive over view of the arguments for and against the doctrine.
While not the focus of his sermon he does spend some time on Mormonism and its rejection of the Trinity (He calls it polytheistic or tr-theistic). I don’t think he in any way slanders Mormonism, but he doesn’t pull any punches either concerning anyone who claims to be a non-Trinitarian Christian.
One aspect of Trinitarian doctrine that I think I have overlooked is why do we consider the Holy Spirit to be God. I was grateful for his attention to detail.
Video Link with a video Q&A