Are non-muslims allowed to visit Mecca?

I've heard that Mecca was closed/forbidden to non-Muslims... I've also heard that the first is a myth, and that non-Muslims (mostly thinking of foreign tourists) may very well enter Mecca -- but that there perhaps aren't much to do for non-Muslims in Mecca... What is correct? Is Mecca open for non-Muslims (tourists) or not? (Link to some official source is especially welcome.) I'm thinking more current enforced Saudi law, than tradition or the Quran...