The Saudis along with Libya supply about 60% of the foreign fighters that go into Iraq to fight against the U.S. occupation and to fight Shiites too. The Saudis just bought $20 billion worth of U.S. weapons. They sell the U.S. light crude oil to refine. They defend the U.S. dollar (somewhat). They keep a dictatorial lid on those within their kingdom who would not be so cooperative with the U.S. Empire. They help to keep Islam split into factions. They take kickbacks for business deals. They are a card (along with Egypt and Jordan and somewhat Pakistan) to play against the Jews in Israel. The list goes on.
The Iranians don't cozy up to the U.S., because the U.S. and Britain stepped on them in a 1953 coup that the Iranians haven't forgotten. The Saudis too have been stepped on, but they suck it up more than have the Iranians. Iranians overthrew the U.S. puppet dictator, and the U.S. imperialists have not forgotten for a second.
The Iranians are friendlier with Russia and China. They buy Russian and Chinese weapons. They trade oil in Euros now too. They just won't come under the sphere of influence of the U.S. They refuse to be dictated to.
The Israelis are being used by the U.S. (even while Israel uses the U.S.) as a front of sorts for U.S. longstanding hostility toward the breakaway Iranians. The Empire has a long memory even if it citizens are conditioned to have short attention spans.
Also, Shiites are a minority in Islam. They are dwarfed in numbers by the Sunnis. This makes it easier to pick on them without a larger Islamic backlash.
Even though the Saudis are sending huge quantities of foreign fighters into Iraq to fight the U.S. and the Iranians are being extremely careful not to give the U.S. any ammunition against them as they continue their nuclear-energy developments, the U.S. still wants to attack Iran and remain silent toward Saudi Arabia. However, once the U.S. has attacked Iran, who's next?
The U.S. is throwing out the steering wheel with its recent economic activities with its home-loan crisis. It will have no choice but to upset the entire economic world to plunder it after militarily destroying much of it if America is to survive as an Empire. They are also banking on the world not wanting to lose all their perceived equity in their U.S. Dollar holdings. Who would take their dollars? If the U.S. tanks, who would consume their products?
The Chinese have been shown what the U.S. can do to them if the U.S. stops buying from them. The U.S. recently started forcing the Chinese to recall many products. No one else can replace the U.S. market for Chinese goods.
Of course, there are many Globalists who couldn't care less about America just so long as those Globalists have theirs. They'll just hedge their bets on insider information and walk away that much richer in diversified investments, currencies, metals, etc.