The political interest has changed, obviously, with the end of the Cold War, and the economic interest is affected by the advent of North Sea, Russian, and other oil sources. The relationship will naturally be modified or modify itself as a result.
In addition, however, the Saudis have found it useful to use the U.S. as a foil to deflect criticism from their own political challenges and their own less than open-handed economic aid policies. This had to be overlooked when its upshot was minor and its value offset by the interests mentioned above. The interests have changed, and the upshot has grown from a local xenophobic resentment to placing U.S. citizens as the focus of a violent international hate campaign, culminating in the events of 9/11. There are, in addition, fissions appearing in the ruling family in the matter of succession to the throne. All of this together means simply that the old world is over, and those who recognize this quickly enough to adapt will be at a distinct advantage over those pining nostalgically for the way things used to be.