Joseph Fielding Smith said something about it in one of his works "Answers to Gospel Questions" most of these were not answers given when he was a prophet but as an apostle. His opinion of it was that the Son's of God refered to the Priesthood holders of that day.
I love this! Great topic Lds_forever! I had been stuck on these verses for the past week and I found a few things that kind of clear a couple things up. First, the Hebrew word here for giants is nephilim which apparently derives from the verb naphal, "fall". This could lead to the translation closer to "fallen ones". In Answers to Gospel Questions (the quote that Brian eluded to) Joseph Fielding Smith said: "The fact was, as we see it revealed, that the daughters who had been born, evidently under the covenant, and were the daughters of the sons of God, that is to say of those who held the priesthood, were transgressing the commandment of the Lord and were marrying out of the church. Thus they were cutting themselves off from the blessings of the priesthood contrary to the teachings of Noah and the will of God." So "fallen ones" should refer then to those who fell out of the church.
Ellis T. Rasmussen, who wrote "A Latter-Day Saint Commentary on the Old Testament" notes that "rather than persons of gigantic stature; the Greek Septuagint rendered the Hebrew word as gigantes for reasons unknown".
Seeing as these verses and verse 5 give the reason for the flood, it would make sense that Moses would write about the dangers of taking lightly the marriage ordinance. Hope that helps!
I was introduced to the book of Enoch through "Teachings of the Pearl of Great Price" by Hugh Nibley. I followed up by ordering his paper "which was read at a gathering of ministers and priest (a regional meeting of the Society for Biblical Literature in 1974) This thesis is printed and can be ordered from FARMS. The title is THE Book OF Enoch as a Theodicy. In it he makes many fine points. Also I have printed from BYU.EDU FARMS Hugh Nibley "Enoch the Prophet." Which is directed more to the LDS membership. So I would sagest for those who want a deeper understanding on the book of Genesis and the fallen angels a though study of these works would be of great help.