I was infuriated when I first read that. After reflecting upon a lot of discussion in the blogosphere and reporting in good Catholic newspapers, I realized that the Pope is correct. If you and I both know a man, and we have vastly different understandings of him, does this mean that we know two different people? Of course not. One of us is likely to be misinterpreting a lot. I believe that to be the case of the Muslims. They worship the God of Abraham, but due to the prophet they follow, well ... they're doing it wrong. This is similar to Paul's 'unknown god' preaching.
There is only one true God, but to lend validity to Islam and say Allah is the same as Jehovah is accepting the lies of their prophet.We must stand our ground.