Here's an interesting feature of iPhoto that I'd never heard of before, courtesy of Mac OS X Hints. While using Edit mode in the app, you can drag two fingers on your MacBook's touchpad (or your Magic Trackpad) to control the various edits with better precision than just dragging around a mouse. You can zoom the crop feature in or out, or even control it manually by dragging two fingers around. And you can rotate the photo in straighten mode just by rotating two fingers around the trackpad.
There are more of these little gesture features hidden in Apple's iApps (like a three-finger swipe in iPhoto to move forward/backward through photos), and I'm sure we'll see a lot more of these pop up as we move closer to Lion and beyond. That touch interface is so intuitive and easy that it's no surprise Apple is already implementing it everywhere that it makes sense to do so.