In Exodus 23:20ff, God, while speaking with Moses (cf. 20:22), declared - "Behold, I send an Angel before you to keep you in the way and to bring you into the place which I have prepared. Beware of Him and obey His voice; do not provoke Him, for He will not pardon your transgressions; for My name is in Him. But if you indeed obey His voice and do all that I speak, then I will be an enemy to your enemies and an adversary to your adversaries." The point I want to stress here is that the Angel of the LORD is distinct from God the Father (whom I believe to be speaking to Moses), yet we observed in prior lessons that this Angel is deity or divine! So, based on my present understanding of the Godhead (cf. our lessons from April 13th and April 14th, 2005), if the Angel of the LORD is divine, He must be one of the three Persons of the Godhead (either God the Father, God the Son, or God the Holy Spirit, as we commonly refer to them). If we remove the first option, the more likely choice that remains is that the Angel of the LORD is the Being we know today as Jesus Christ (in a different physical form, of course). It seems unlikely that the Holy Spirit would assume the form of an Angel since nowhere else in the Scriptures is He stated to have taken on a physical form. The same cannot be said regarding the Second Person of the Godhead.
There is one other argument worth mentioning on this subject. We noted previously in Exodus 14:19 - "And the Angel of God, who went before the camp of Israel, moved and went behind them; and the pillar of cloud went from before them and stood behind them." However, Exodus 13:21 states that it was the LORD Himself who did this. Furthermore, Paul wrote in I Corinthians 10:1-4 - "Moreover, brethren, I do not want you to be unaware that all our fathers were under the cloud, all passed through the sea, all were baptized into Moses in the cloud and in the sea, all ate the same spiritual food, and all drank the same spiritual drink. For they drank of that spiritual Rock that followed them, and that Rock was Christ." Here Jesus is identified as the "spiritual Rock" that followed the Israelites during their long journey from Egypt into the Promised Land. Speculation abounds as to what exactly is meant by this phrase. Some believe this refers to a literal rock from heaven that rolled behind the Israelites and provided water as they journeyed. Others, including myself, are more inclined to accept a less literal perspective here. In either case, Christ miraculously provided water (and hence stability like a Rock) for the Israelites on at least two occasions. He was undeniably there with them, though in what sense it is impossible to know for sure. Perhaps there is a connection between the "spiritual Rock that followed them" and the Angel of the LORD who seemed to move with the pillar of cloud and fire.
Therefore, in closing, I believe that the angel of the LORD of the Old Testament is the preincarnate Christ. I believe that the Second Person of the Godhead was active during those 4000 years between the creation and His birth as Jesus of Nazareth (cf. John 5:17). Let me leave you with this disclaimer, however. Due to the difficultly of this subject and the fact that there is some unavoidable speculation in my argument, I am not dogmatic regarding this subject. I can see how others may be less willing to connect the identity of the Angel of the LORD to the preincarnate Christ. It is certainly not a significant point of doctrine either way (or the Scriptures would have been clearer on the matter). Nevertheless, it does make for an interesting study from God's word.