Here’s a problem you’ve probably encountered while trying to show someone else a picture in the iPhone’s Photos app. As you pass the phone around, the accelerometer keeps flipping the image from portrait to landscape to upside-down landscape and back again. Annoying, right?
As it turns out, there’s an easy way to fix this (at least on iPhone 3.1.2, which I'm running): just tap and hold on the image for a second or two and the phone will lock it into position. This might cause a “Copy” bubble to pop up, but it will disappear after a moment, and the image will retained locked, no matter which way you flip the phone. To unlock it, just tap the image again and you’re done.
I only get to know this now after using the iPhone for two years now. You see, there was no app for that.