Once again James Chastek (who has the advantage over most other people that takes advantage of the wisdom of St. Thomas Aquinas) nails it on an issue on which a lot of nonsense is being written:
Hypothesis: Asking whether Muslims and Christians believe in the same God is the same as asking whether geocentrism and heliocentrism are descriptions of the same universe.
The question is not best seen as followed by an instruction to check the yes or no box. If you’re asking whether contrary theories to explain the same fact are about the same fact, then the answer is (analytically) yes. If you’re asking whether contrary theories to explain the same fact are the same (i.e. not contrary) then the answer is (again, analytically) no. Rather, the question becomes interesting when we ask what relation contrary accounts have to the thing they are accounting for.
My only question here is why we couldn't make the same argument about, say, about Mormonism. It seems to me that Mormonism is a different matter than Islam in this regard. While Islam is a Christian heresy—in that it came out of Christianity and retains enough of the Christian conception of God's essential nature to be considered a wrong belief about the same God, Mormonism seems to deny every essential feature of the Christian God, constituting a conception of God completely alien to the Christian one.
Of course, this assumes a legitimate distinction between God's essential features and his accidental ones, but I believe that could be done.
In any case, Chastek's post is here.