The iPhone (and iPod touch) automatically switch the screen between landscape and portrait mode when you tilt the device. This can be annoying if you want to read a web page or mail message when reclining in a horizontal posture on your side (while, say, lying in bed).
But you can 'lock' the iPhone/iPod touch in landscape mode by turning it upside down slowly. This is perfect for reading while lying down.
Hold the device in an upright position (default portrait mode). Tilt the device ninety degrees in either direction (switches to landscape mode). Tilt the device another ninety degrees in the same direction, so that it's now upside down. The screen orientation stays in landscape mode, and the text is now aligned for reading on your side in a horizontal position.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090707145700601