While I also am not an authority on all things LDS, I have develed into the faith and doctrine to some extent, and I'll lend my backing to
UndercoverElephant's post of this belief. There are a lot of misunderstandings and minsconceptions about the LDS faith, but this is not really one of them. UE has provided most of the quotes that I would have offered already. The doctrine of eternal progression, that man is capable of advancing by stages, and what is learned at one stage is carried forward to the next, is not a new one. Men cannot become God, Himself, but they can become god-like in responsibility and, by extension, dominion over others so long as they follow the spiritual teachings and rules.
Bob, I'm more than open to being corrected on this issue, and I assure you that in no way, shape or form am I laying any kind of argumentative trap in order to receive your response. In fact, I'd be more than willing to continue this discussion via PM in order to get your take, evidence, etc. I'd rather be corrected in this regard, than go around continuing to make incorrect statements. I don't know if it will do any good, but my hand is extended on this issue.
ETA: But back to the OP, this aspect of the LDS faith does not make them polytheistic. The church does not claim, as far as I know, that anyone from Earth has progressed to god-hood, and certainly doesn't teach praying to such folk. There is a polytheistic element to the LDS faith in regards to the God the Father, God the Son, and God the Holy Spirit, three seperate beings with one purpose, each of whom can be prayed to.