Saying that Jews (and Muslims) and Christians worship the same God is both true and false.
It is true in that the God of natural revelation is God. I had a conversation with a Muslim at the jail awhile back. We started talking about how we connect with God. His way, of course, was through the five pillars of Islam; I talked about Jesus and his work. Now, this conversation would only make sense if we were talking about roughly the same God. Now, from what I'm reading here, I should have started off by telling him, "You realize that you Muslims worship an idol, right?" I'm sure that would have gone over well. I think the same would be true in Jewish evangelism. I don't think I'm trying to get them to go to a "different God." Rather, I'm trying to get them to see that the only way to God is through Jesus.
At the same time, this statement is false, since they deny that Jesus is the Messiah. Since they reject Jesus, they really don't worship God.
I think the problem most of us have with "Jews, Muslims, and Christians all worship the same God" is that it gives the picture that we are all trying to get to the same goal and there are many equally good paths. I think that's the wrong metaphor. I prefer that of a maze. We are all trying to get to the same goal, but only one path will get you where you want to go. Everything else is a false path and a (literal) dead end.
One final thought. Pieper, in his first volume, pp. 9ff., points out that there are two religions in the world: that of the Law and that of the Gospel (p. 10). On p. 12, he says that the "nature and character of Christian worship difference radically from that of the worship practiced in all other religions. Christians worship God as the God who has bestowed grace upon them... To non-Christians the worship and service of God is the performance of a 'religious duty' dictated by the fear of God's wrath."
The implication to me is that Christians and non-Christians worship the same God, but one under the religion of the Gospel and the other under the religion of the Law. (And he makes it clear in a footnote on page 9 that he is including Judaism and Islam under the religion of the Law.)
All I'm trying to say is: let's be careful in our wording here! It can get quite messy otherwise.