The official Mormon doctrine, as far as I know, is that yes, Jesus has existed as God from all eternity. That's the easy answer.

Now, for the more in-depth portion of the answer.

Basically, I believe we have a fundamental problem in trying to conceive of what eternity means. My opinion, as I have stated elsewhere, is that God and Christ exist outside of what we call "time". As far as our universe goes, and as far back as we can understand time (to the big bang, I guess), God has always been God, and Christ has always been Christ. Note that Christ's nature has changed a bit, since he received a physical body as part of his mortal experience, which he maintained after the resurrection (see Luke 24:39). Nevertheless, he was undoubtably part of the "godhead" (comprised of the Father, the Son, and the Holy Ghost) before his birth as well.

The creation as I see it, in addition to forming the Earth, also refers to bringing this universe into existence (as a physicist, I assume this was done via the big bang). I believe, and I think Mormon doctrine implies, that Jesus and God the Father were not created in the big bang, but rather already existed and were the motivating forces involved. The same can be said of our own personalities-- we were not created in the big bang either, and thus are "co-eternal" with God.

Now, part of the confusion may be terms like "First-born". We believe Jesus was the "First-born". I believe that refers to the process of placing an "intelligence" (a concept which we do not understand well) into a "spirit body". Jesus was the "first" (whatever that term may mean with respect to a process which may have occurred outside of time) child of God the Father to be placed into a spirit body. When this happened we do not know-- it may have been outside of our universe altogether.