I've heard of RGB Sub-Pixel Rendering.
I'm wondering does the IPhone 3 have this capability (or is it that each pixels are only 1 color) ?
What about the IPhone 4?
They don't. The native resolution of even the original iPhone screen was high enough that there would be little added benefit.
Also note that each orientation of the screen would require a different set of subpixel rendering calculations. So it would also be more complicated to implement.
Combined, there's no advantage to use subpixel rendering on an iOS device.