The answer to this question may depend on whom you ask. But, my opinion is that we worship God, Jesus, and the Holy Ghost. However, we pray only to God the Father.

I think the problem is the ambiguity of the word worship... what does worship mean? It seems like even LDS church leaders have disagreed.

For example,

George Q. Cannon (former apostle) says something very close to my opinion:

"The oneness of the Father and the Son. Now, can you conceive of a oneness more close, more complete than the oneness that exists between the Father and the Son? It is impossible for the human mind to get the faintest idea of any difference of opinion or expression or action between the Father and the Son. We worship them as one God—not three Gods, not two Gods but as one God. The Father and the Son are the two personages of the Deity, with the Holy Ghost as their ministering Spirit or agent. We worship them as one. We do not separate them in our thoughts and in our feelings. . . .

"While they are two Personages, they are but one—one in feeling, one in thought, one in mind, one in everything, in fact, in every direction in which their power is or can be exercised. And in all the records that have come down to us from the Son of God there is one thing that stands out clearly and prominently throughout all the teachings and acts of our Lord and Savior Jesus Christ and that is His entire devotion to the Father, His complete submission to the will of the Father and His oft-expressed desire to know and to do that will. It is wonderful the submission that the Savior manifested in His life, when we think of His great dignity and the position that He occupied. . . ."

However, Joseph F. Smith (former president of the church) expresses a different view:

"It [God's work] is the power of God unto salvation, and I want my boys and girls to take my testimony upon this point. And yet, while we give the honor and glory unto the Lord God Almighty for the accomplishment of his purposes, let us not altogether despise the instrument that he chooses to accomplish the work by. We do not worship him; we worship God, and we call upon his holy name, as we have been directed in the gospel, in the name of his Son. We call for mercy in the name of Jesus; we ask for blessings in the name of Jesus. We are baptized in the name of the Father and of the Son and of the Holy Ghost. We are initiated into the Church and Kingdom of God in the name of the Father and of the Son and of the Holy Ghost, and we worship the Father. "

I think some more recent statements by church leaders share the first view (that we do indeed worship Christ). I don't have any citations for them right now though.

In any case, the Book of Mormon is very specific about the people worshipping Christ when he visited the Nephites.

3 Ne. 17:10

10 And they did all, both they who had been healed and they who were whole, bow down at his feet, and did worship him; and as many as could come for the multitude did kiss his feet, insomuch that they did bathe his feet with their tears.