I think shipping times probably vary by location and what you order. I live in Austin, TX and pre ordered Iron Man. I selected the free super saver shipping option and I got in on Wed, the day after release.
I always select free shipping. My experience with Amazon generally has been, the more in advance that you pre-order a title, the sooner they'll ship it. I figure that, within each shipping cost category, Amazon probably ships early pre-orders first.
Just wanted to place this public service announcement concerning a change in Amazon's price guarantee back on 9/1/08. They no longer provide refund/credit if they drop their price within 30 days like before.
...Steve "People should not be afraid of their governments. Governments should be afraid of their people." - V