The serpent that tricked Eve was never actually referred to as the Devil. The exact quote is here (Genesis 3:1-5)
"Now the serpent was more subtil [sic] than any beast of the field which the Lord God had made. And he said unto the woman, Yea, hath God said, Ye shall not eat of every tree of the garden?
And the woman said unto the serpent, We may eat of the fruit of the trees of the garden:
But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall nto eat of it, neither shall ye touch it, lest ye die.
And the serpent said unto the woman, Ye shall not surely die:
For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil."
See, no devil. The only thing mentioned is the serpent. This leads me to believe that the story has been modified over time, whether it was 10 years after it was written or 1000. It appears to have happened sometime. Now, I'm not going to go flip through the Bible and find every page that the serpent is referred to as the Devil, but I can assume that because the Bible is in a mostly chronological order (though, the Epistles of Paul were written before the Gospels), that any mentioning of the Devil/serpent combo would have been an afterthought.
That's just my thinking on the matter.