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I'm still very new to programming in OpenCV, so take this with a grain (kilogram) of salt.

The dimensions, according to a non-authoratative source, is 12 in X 6 in. This is the real size, but it gives us an accurate model in which to work with.

Now, our camera (in your case, an iPhone) is simply a directional point camera with a rectangular cone field of view. From an arbitrary point, the 12x6 rectangle will look like a parallelogram. So what we ideally want is to make a matching parallelogram over the boundary of the license plate, and then map the parallelogram to the appropriate rectangle. Then we can OCR this intermediary image and output the appropriate letters and numbers.

1. Capture picture.
2. Best-fit a parallellogram over the mostly solid color boundary to find the license plate.
3. Map the parallelogram to the 12x6 license plate
4. OCR corrected plate picture.