I think we really need to define our terms.
1) Definition of mutable: http://www.merriam-webster.com/dictionary/mutable
2) Definition of immutable: http://www.merriam-webster.com/dictionary/immutable
According to Merriam-Webster, that which is mutable is "prone to change, inconstant, capable of change or of being changed, capable of or liable to mutation."
If one agrees that God is not "inconstant" or "liable to mutation," then one must conclude that God is immutable.
1) Definition of mutable: http://www.merriam-webster.com/dictionary/mutable
2) Definition of immutable: http://www.merriam-webster.com/dictionary/immutable
According to Merriam-Webster, that which is mutable is "prone to change, inconstant, capable of change or of being changed, capable of or liable to mutation."
If one agrees that God is not "inconstant" or "liable to mutation," then one must conclude that God is immutable.