I had Samsung focus before my lumia 900, and at first I thought the same thing that the camera button is not waking the phone up.
but the difference is that the lumia 900 need longer press of the camera button that the focus,
"YOU NEED TO KEEP IT PRESSED UNTILL IT VIBRATES" to let you know that it detected your button press, so basically upi need to keep the button pressed untill the camera activates.
if you have used another wp7 model, like me a focus, it does not work the same , on focus it would onlu need a FULL press , you do not need to keep it pressed for long. but on the umia 900 if you let the button go before it vibrates, then it's going to be hit or miss whether it works.