This might be a strange question. I have an app on my phone (HTC Desire) that has all sorts of photography related resources, likecalculator, reciprocation calculator, flash , color temps chart, sunrise sunset times, moon phases etc etc. It is very useful!
Anyway, I just found that it has a grey card function, where it gives you a blank grey screen. Now this just doesn't seem right to me. Wouldn't the screen brightness of your phone screw with that and your WB would just be rubbish? I'm just not sure it is something useful? Any thoughts?