Nearly one quarter of US oil imports depend on the middle east. And the idea is to gain both cheap supply, and consistant, to secure long term success in a heavily oil dependant country. Since it isn't viable politically to take anyone else oil fields, Iraq was a pretty damn good heist in terms of plain oil value. I'm still not convinced it was more of an offensive move versus China. And I think that Bush's strategists were aware of the short term rise in oil prices, but gambled on a longer term hold of that areas oil exports.
I think this also ties into why they completely ignored the fact the attacks on the World Trade centres were connected more to the Saudi's than the Afghani's. Saudi Arabia is a very powerful nation and both it and the US profit more from. Iraq hasn't traded with the US heavily in years - but they could. It's a great and profitable venture just awaiting the right opportunity. Now that opportunity is gone, and an american-heavy deal is going to be cut in it's place. The economic rewards of such a move tie very directly with companies closely related to Bush's administration, and that pretty much convinces me the move wasn't some kind of altruistic future-looking defense move but almost purely a greedy money grab.
Yhorian