I always just assumed the only reason it has an icon on it, is so you can see where it is easier, its not like it needs distinguishing from other buttons, and the icon shape i assumed was a rounded square to match the rounded edges of the phone itself,
Who knows... Maybe you're reading too much into it, or maybe apple really did put that much thought into it.
EDIT: you can see in the image below it is not actually the same as an app icon, the corners are more rounded.
Image
but you can also see below that the corners do not match up with the rounded edges of the phone either.
Image
I tried this with an image of the original iPhone too and it still didn't match up.
So who knows...