What is a major reason for U.S. interest in Saudi Arabia and the Persian Gulf?

Oil most certainly, since Saudi Arabia, Iraq and Kuwait combined hold almost 40% of the world's known oil reserves.  In addition, we have a major interest in Saudi Arabia in that, as their best customer, we have a stable, mutually beneficial relationship.  They act as one of our only Arab allies in the region, have allowed us to use the Kingdom for a military base at times, and have, at least in some ways, given us aid in the war on terror.

Remember that Osama bin Laden is Saudi Arabian, and al-Qaeda was born in that country.  So the government of that nation have just as much interest in seeing him captured and his terror network dismantled as the US does.