Ok, just did a quick + dirty test. Profiled the GPU utilisation on my 4S as I was doing some profiling on it anyway. Played around a bit in the system, scrolling around, opening + closing stuff, resizing the screen in the browser, all the usual stuff. GPU usage was generally 5-10%, and peaked at 20% (this was extremely rare).
Stretching that figure out a bit and using the peak, it could handle 5x more screen area. That's enough to drive a retina iPad - and the 4S GPU is under clocked. You'd be hitting near 100% utilisation now and then, but it would be rare. It'd generally be hitting 25-50% utilisation.
So yes, the A5 would handle a retina iPad display for the vast majority of non-game apps