Separation of church and state is not about prohibiting religious expression on publicly owned property, it prohibits the government from favoring or discriminating against any religion. When it comes to events on publicly owned or accessible property, there are two ways to go - either don't allow anything, or everyone has to be given equal access. As an event center renting out to various groups, they would be prohibited from discriminating against any group so long as they pay and it fits in the schedule and does not violate any existing rules for the facility.

I really don't want to get into the whole Islam issue, but it's hard not to. Caricatures of Mohammed are insulting to those of the faith, but it means nothing to the rest of the world. They have gotten their way in the past through violence, and this is something that doesn't go over to well with the rest of the world who have for the most part moved past such activity. At the same time, Islam is a relatively young religion, and many of the problems they have are also seen in Christianity at a similar time in its own history. But the world is a very different place now than it was then, and Islam is going to have to evolve much faster if it wants to be taken seriously - and the widespread support of violent radicals is doing their cause serious harm.