I've seen the question tossed around before, and James Taranto says the following, in the context of quoting President Bush:
Last week in Britain, a reporter asked President Bush if "Muslims worship the same Almighty" that he does. Bush replied: "I do say that freedom is the Almighty's gift to every person. I also condition it by saying freedom is not America's gift to the world. It's much greater than that, of course. And I believe we worship the same god." The Washington Post reports that the president's ecumenism prompted a kerfuffle among evangelical Christians. ...
Bush is right. Christianity, Islam and Judaism are all monotheistic religions, united in the belief in a single God. (Muslims often call God by the Arab name Allah, but then so do Arabic-speaking Christians and Jews.) The three religions conceive of God differently, and Muslims and Jews do not share the Christian belief in the divinity of Jesus Christ. A Christian may well believe that Islam's conception of God is wrong, but if you believe in only one God, it makes no logical sense to describe a fellow monotheist as worshipping a "different" God.
To an unbeliever, that may be a perfectly satisfactory answer -- since he wouldn't believe in any God, the details are inconsequential. It's true that as a monotheist I believe there is only one God, but it doesn't follow that anyone else who is also a monotheist worships the same God I do; the alternative is that they don't worship God at all, but rather a construct of their own imagination. For example, someone who woships a rock or a tree and claims it is the one and only "god" may also be a monotheist, but the characteristics of their "god" are entirely different from the characteristics of mine; we may both be monotheists, but at least one of us is wrong in believing that our god is the one and only.
Similarly with Muslims and Christians. Both are monotheists, but the two concepts of "god" are so completely divergent that they cannot both be true, and both "gods" cannot exist as conceived. At least one of the religions is wrong (and both think it's the other guys', whereas unbelievers think it's both).
Typically, only unbelievers (and functional unbelievers) are willing to make the claim that Jehovah and Allah are "the same". Why? Because they don't believe in either, and it's convenient and "enlightened" to lump everyone together. Why quibble about differences between two imaginary beings?
In the next day's Best of the Web, Mr. Taranto continues:
A Bush supporter's conception of Bush's "constitutional makeup" is utterly at odds with that of a Bush hater. Not all conceptions about Bush are equally true; Paul Krugman, for example, is totally wrongheaded, while this column generally is the model of verity. But whether Krugman is writing about him or we are, George W. Bush is the same man.
By the same token, to say that all monotheistic religions worship the same God is not to say that they are all equally valid. Indeed, since Christianity and Islam make competing claims about the nature of God, it would be logically incoherent to argue that both are true. Yet to say that they worship the same God does not contradict either religion's claim to be the one true faith. As to which religion is true, that is beyond the scope of this column.
Mr. Taranto is still not seeing the big picture, because he isn't recognizing what Christians and Muslims see to be fundamental attributes of their gods.
To carry my rock-god and tree-god example further, if I believe that some specific rock is the only god, and you believe some specific tree is the only god, it's meaningless to say that we both believe in the same "god" just becuase we both believe there's only one. If you're right, then the rock I believe is god is really just a rock and my god doesn't exist; I'm so fundamentally wrong about tree-god's nature that I'm worshipping something entirely different, something that isn't real.
The belief that there is only one god is one fundamental characteristic of that god, but not the only fundamental characteristic.
The Muslim claim that they worship the "God of Abraham" is fallacious; the origin of the Muslim religion can be seen in its modern symbolism: Allah was originally the fertility-/moon-god of Muhammad's tribe, and Islam carries the crescent moon symbol even still. In my (limited) experience, most Muslims are not aware of this aspect of their history, but it is pretty well supported by official Islamic historical records.
Donald Sensing gives more details, with all of which I concur. ["with all of which I concur"? ick -- Ed.]