As I have argued before, the invasion of Iraq, coupled with the invasion of Afghanistan and the turning of Pakistan completes what is essentially an encirclement of Iran. Further, as a look at a topographic map will tell you, Iraq provides far easier access to Iran's interior than other alternatives.
Saudi Arabia may contain sources of funding and even human assets for terrorism, but Saudi Arabia itself is not, in my view, a hard target to attack if American protection is removed. There is not much need to encircle it. Iran, however, is a much tougher nut to crack, from every direction except the west.
Further, the fact that some Saudis have provided financing of terrorism and for Islamic fundamentalist schools does not mean that they are enthralled with Iran becoming the biggest dog in the neighborhood. Underneath it all, they are realists. At some point the Saudi royals will have to rein in the more extreme of their brethern (literally). The removal of U.S. forces from the Kingdom should help quell anti-American resentment among the more moderate elites. Saudi Arabia has enough internal problems to keep it busy for the foreseeable future.
Update: Here's a link to a relief map of Iran.
Update2: Lots of good comments at Professor Bainbridge's site, but here's good quote from The Owners Manual
Most of the 9/11 perps were Saudis, providing legitimacy to reacting against Saudi Arabia. But we didn't because... they have too much oil. Instead, we attacked Iraq... who only has a whole lot of oil.
Remaining steadfast in the task of cramming democracy down on people who increasingly resist it at the cost of a thousand of our finest countrymen is difficult enough without asserting that it is all about containing an erstwhile ally. That sounds like war by State Department.