What's slightly sneaky about this is there's nothing apparently different from an end-user's perspective at first glance. Clicking "purchase" or "buy now" used to automatically place items in your shopping cart if you had that preference enabled, but now the behavior has been completely altered – now all purchases on the iTunes store are 1-click if you click the "Buy Now" button, and there's no way to cancel them once they start. So whether it's a $1.29 song or a $129 iPhone app, if you click that "Buy Now" button, you're getting charged.
If you want to emulate the old "shopping cart" purchase behavior, you have to instead click the arrow to the right of "Buy Now" and select "Add to Wish List."
Save your wallet! Always click the arrow!
Update: If you had items in your shopping cart before updating to iTunes 9, they won't have disappeared. All of the items that used to be in your shopping cart in iTunes 8 or older should have migrated to the new Wish List in iTunes 9.