Hebrews 1:5 is commonly used to attack the Trinity and attempt to refute the "eternal Son" doctrine because it says, "TODAY I have begotten you". Does this prove the Son came into existence at a point in time? Created by the Father?

Aside from the points I made in this video, I think we shouldn't read too much into the verse; things that were never intended by the author. It's possible that the author was making the simple point in Heb 1:5 that Christ is the unique, begotten Son of God and is therefore greater than the angels (that there is a unique Father-Son relationship).

The following are the best possible ways to interpret Hebrews 1:5 (or a mixture):

1.) "Today" refers to the coronation or inauguration of the Son in His role as King and Messiah (Psalm 2:6).
2.) "Today" refers to the day of His resurrection (Acts 13:33, Rom. 1:4).
3.) Jesus is simply being openly declared to be the Son of God in time like He was at His baptism, transfiguration and resurrection.
4.) Jesus is simply being declared as the unique, begotten Son of God and is therefore of the same essence as the Father (John 5:18).
4.) "Today" refers to the the eternal decree of God rather than a decree in time.
5.) The verse refers to the incarnation and humanity of the Son as human Messiah and representative of God.

"God made the worlds "through" His Son... eternal Son? (Hebrews 1:2)":

http://www.youtube.com/watch?v=nHuDYo_wepw

John MacArthur also has an interesting interpretation of Hebrews 1:5/Psalm 2:7:

"It is now my conviction that the begetting spoken of in Psalm 2 and Hebrews 1 is not an event that takes place in time. Even though at first glance Scripture seems to employ terminology with temporal overtones ("this day have I begotten thee"), the context of Psalm 2:7 seems clearly to be a reference to the eternal decree of God. It is reasonable to conclude that the begetting spoken of there is also something that pertains to eternity rather than a point in time. The temporal language should therefore be understood as figurative, not literal."

And:

"Therefore, the "begetting" mentioned in Psalm 2 and its cross-references has nothing to do with His origin... But it has everything to do with the fact that He is of the same essence as the Father."

Theologian and commentator, Matthew Poole, agrees that Hebrews 1:5/Psalm 2:7 refers to the Messiah's inauguration and is connected to the resurrection of Christ. He comments on Psalm 2:7:

"And this day or time Christ might very well be said to be begotten by God the Father; partly, because the resurrection from the dead is in Scripture called a regeneration or second birth, Matthew 19:28, as well it may, being a restitution of that very being which man received by his, first birth, and that by the peculiar and mighty power of God; partly, because in this respect Christ is called the first begotten of the dead, Revelation 1:5"