Theory best summarized by the couplet
As we are now, God once was,
As God is now, we can become
Commonly used by anti-mormons to scare people away from the Mormon Church. A common claim is that the Church hides this idea from its members and outsiders. In fact, this isn't true at all. Most mormons I know are very familiar with it.
The basis of the theory is that Mormons believe we are literal children of God, and therefore can inherit all the characteristics of our Heavenly Father. Something else to consider the statement (paraphrased), "Be ye therefore perfect, even as your Father in Heaven is perfect". Would God give us a commandment that cannot be followed?
I named this the God-Man theory since it's not solid Mormon Doctrine. It has never been fully explained, and is something of a fringe belief.
I always thought this was peculiar to Mormons but in fact it's believed by many Christians such as C.S. Lewis and St. Augustine.