I visited an AT&T and Microsoft Store and inquired about the problem. Every Nokia Lumia 900 on display and even the store employee's own phones exhibited the "works when it feels like it" camera button problem. The store employees had no explanation for it.
Through my own fiddling with it I found the only way to get the camera button to work every time is to press the power button first to wake up the phone, then press the camera button. It is a pretty easy work-around.
I also found that if you press the camera button slow enough to feel both clicks, or press the camera button twice quickly, that can work too, but not all the time...
As far as I know Nokia has not acknowledged this problem yet but I hope they will soon and that a software update will correct it. Does anyone know if Nokia will be fixing this problem?
Solved! Go to Solution.
UPDATE: I just installed "Tango" update from AT&T which included updates from Nokia. This problem has been fixed! Holding the camera button down while the phone is asleep now brings up the camera.
I've also seen this. But there are mobile apps developers who can actually develop those apps which could be used as an alternative of this.