As a person who's used Windows Phone since the very first version I know that one of the missing features was the ability to take screen shots! This was infuriating for developers who wanted a screen shot of their app working to place in the app store (then called marketplace). What made it even worse though was the simulator also lacked the button! Thankfully an update to the simulator fixed this, but the phone was still lacking.
Until..... I don't know when, but the other day I accidentally did it and now know that screenshots are taken by holding power and pressing the windows key before the slide to power off box appears.
I've also hear in WP8.1 this will be replaced with power button and volume down as the requirement for physical back, windows and search buttons is to be removed.