As a tech blogger, I use the iPhone's screen capture function (power button + home button) on a daily basis, which makes it all the more frustrating I didn't think of this first: using it to crop photos.
After reading this post on iPad Insight, it seems so obvious. Simply pinch to zoom in on an image, position it just so, and hit the power button and home button to capture the newly cropped photo as a new image in your camera roll. Done and done. It works on both the iPad and iPhone (and presumably the iPod Touch, an Apple device I do not own).
iOS 5 added a number of editing tools to the Photos app, including cropping. If you don't need to change the aspect ratio of an image, however, the screen-capture method is faster. And unlike cropping a photo using the edit tools in the Photos app, the screen-capture method saves your cropped photo as a new image instead of overwriting the original.
Well played, Patrick Jordon of iPad Insight. Well played.