All my days are ordained before I was born? Psalm 139:16 (KJV) Thine eyes did see my substance, yet being
  unperfect; and in thy book all my members were written, which in
  continuance were fashioned, when as yet there was none of them.


My understanding of this verse: God has written down the process of how our bodies will be formed. Thus, referring to embryology. 


  Psalm 139:16 (NIV) Your eyes saw my unformed body; all the days
  ordained for me were written in your book before one of them came to
  be.


My understanding of this verse: God has written down how many days I should live on this earth. Thus referring to predestination. This translation is very hard to digest for me.

How should we interpret this verse?  I myself cannot accept the NIV way of translation because that would put God responsible for everything in my life, whether good or bad, and that even my destiny of going to Hell or Heaven is predestined. Some writers like Rick Warren  prefer this NIV way of translation.

What is the view of Mainstream Christian Denominations on this verse?
Foreordination is not the same as predestination.

Ordination in the Bible typically refers to being set apart to the office in the Priesthood, for example, the members of the house of Aaron were ordained to take care of the tabernacle during the exodus and later the temple. 

A great example of someone who did not follow through with his ordination is Kind David. Even though he was ordained to great things he committed great sin and lost his opportunity.

When the bible refers to foreordination it means that God has identified the potential of certain individuals and ordained them to do great things before they were born. That said, someone does not have to follow through with their potential because we were each given what's probably the most difficult gift for any father to give: the freedom to choose.