I would say no.
Its my understanding that a grey card serves two purposes and the iphone would fulfill neither.
First, the gray card is used as a tool to obtain an exposure reading by reflecting available light off of a card that is a constant 18% shade of gray. The iPhone will not supply reflected light. It is back light by LEDs or some such light source, and as such will be worthless for this purpose.
Next, a gray card can be used for a custom white balance. The iPhone's display will vary based on the available light so it will be visible in bright light as well as low light. I would think that the ever changing brightness of the display will wreak havoc on your ability to get a proper and constant reading, and make your white balance all kitty wompus.
I could be wrong though, I frequently am.