It's not so much the oil. Even if the Saudis hated the US they'd still sell their oil on the global market. What we truly love about the Saudis is their cash. Remember:
1. When the Saudis sell oil, they get dollars in exchange.
2. which means they have hundreds of billions of dollars to play with.
3. many of those billions end up being invested back in the US, US companies, or investment banks with strong US ties.
4. they had something like 1 trillion dollars worth of US investments in 2004 (last year I can dig up any good numbers)
Truth is, they own a good chunk of our asses.