Here is an excellent article on THE SONS OF GOD and the Nephilim...
seems to be well thought-out and well presented...

The Sons of God and the Daughters of Men
(Genesis 6:1-8)
By: Bob Deffinbaugh , Th.M.

"I therefore understand the Nephilim to be a race of super-humans who are the product of this angelic invasion of the earth.
This view not only conforms to the biblical use of the expression ‘sons of God,’ it also best fits the context of the passage. The effects of the fall were seen in the godly offspring of Cain (chapter 4). While Cain and his descendants were ‘in Satan’s pocket,’ Satan knew from God’s words in Genesis 3:15 that through the seed of the woman God was going to bring forth a Messiah who would destroy him. We do not know that the entire line of Seth was God-fearing. In fact we would assume otherwise. Noah and his immediate family alone seem to be righteous at the time of the flood..."
http://www.bible.org/page.php?page_id=67