One of the reasons I love this country is our "freedom of religion", however building a mosque on ground zero should not be the point.
911 was not about Muslims (only), it was about all the people affected by a group of misguided individuals. If there is any monument set that must include a religious reference I think it should be one that recognizes all the different faiths of the people who lost their lives on that day or as a consequence of 911.
There were undoubtably muslims, christians, catholics, jews, atheists, buddhists, new agers, etc...
A monument truly reflecting the people is the point, not a faith. And if a monument with faith significance is important, then build one that includes ALL faiths NOT singles out one in an attempt to appease or seem PC. A place that asks that all people, no matter their faith, to come together and honor each other and their faiths.
This is an opportunity to unite people and help them realize that our various faiths have the potential to Unite instead of Divide. Heal instead of Harm.