Mecca is in no way equivalent to Jerusalem, and the Kaaba is in no way equivalent to the Temple during the pre-exilic period. God does not "live" in the Kaaba. God did not "give" the Hedjaz to the Muslims as their Holy Land. There's no denying that Mecca is important to Muslims, but it's not Jerusalem and it's not the Vatican.
Second of all, all Islamic law is based upon case law - that is to say, it is designed to deal with changes in circumstances and new developments - unlike the Torah. Shariah is more comparable broadly to the Talmud, which arose largely to deal with the circumstances of a Judaism deprived of a focus and forced into exile. For that reason, Islam is already in a better position to deal with such a religious crisis than the Jews were when their Temple was destroyed.
Third of all, Brilliant, what makes you think that OBL "can't risk being responsible for the nuking of Mecca and Medina?" Do you really think that's the way he thinks? If you think such a rumor acts as a "deterrant" to the greatest mass murderer alive today, you're sadly mistaken. If he could get the US to nuke Saudi - even to destroy the Holy Cities - he would. It would result in the very sort of Gotterdamerung that he's been trying to bring about all along.
Matt - Islamic eschatology is relatively dull compared to some of the Jewish and Christian apocalypses. They believe in the Day of Religion - the yawm ed-din - or the Last Day - yawm al-akhir - on which the dead will rise from their graves and be judged. Other than that, it's pretty typical stuff. They make up for its relatively stereotypical nature by giving graphic details about Heaven and Hell (Jannah wa-Jahannam). IIRC, something will happen at al-Aqsa Mosque (that is, "the furthest mosque") but it has been a while since I studied this stuff. You may be interested to know that some of the more mystical sects believe that Jesus will come back, abolish the religion founded in his name, rip the crosses from the wall (a bit like Reverend Moon), and personally slaughter every last pig on earth.