When Abraham sees the three men who have come to visit. He recognizes something special about them--something divine--and responds accordingly. When he says, "Oh Lord, if I have found favor...," the Hebrew word used (Adonay) that the ESV translates as "Lord" is a word that is only used for God in the Old Testament. This indicates that he recognized something of a divine nature in one of the men. I believe this is probably one of several times in the Old Testament that Jesus appeared personally to deliver a message. Most scholars agree that there are other occasions when you see the phrase "the angel of the Lord" (instead of "an angel of the Lord") and the individual is addressed specifically as "Adonay" (Lord), that it is God the Son making an appearance.
One specific verse I want to point out:
Genesis 18:12 (ESV)So Sarah laughed to herself, saying, "After I am worn out, and my lord is old, shall I have pleasure?"
No reason. I just think it's pretty stinkin' funny!