Amazon's Kindle iPhone app has a very neat implementation of this. Wehenver the screen adjusts its rotation, a little round lock button fades in, sticks around

Amazon's Kindle iPhone app has a very neat implementation of this. Wehenver the screen adjusts its rotation, a little round lock button fades in, sticks around for a while, and fades out again. If you touch it, the screen is now locked in the current orientation. To see the button again, simply rotate your iPhone to another 90° angle range and it'll fade in again briefly. Works really well, maybe not so great for the small plane scenario where orientation changes all the time making the continuous fading in/out a bit annoying, but great for statically holding your iPhone at an off angle.