I too have have this issue with my 21.5 iMac. I don't have a true fix, but after a month I found a 100% reliable way of getting the system to wake from sleep.
This assumes the computer is already asleep at the start of my procedure and that in the prefs panel you allow the computer to be PUT TO SLEEP with the power button...
1) Click keyboard or mouse to wake computer from sleep. Nothing happens but actually the computer HAS woken up, just not the screen... sorta. In fact if you move less than an inch away from the screen and look at various angles you might see the almost imperceptible hint of what is on the screen.
2) Momentarily press the power button. This will put the computer BACK to sleep.
3) Wait about 10 seconds for the computer to return to sleep.
4) Press the keyboard again, this time the computer will wake normally.
Why this works, or what is happening I certainly don't know. I have reset PRAM, Video card etc. with the apple procedures but it doesn't help. Hopefully a software update fixes this full in the future but this certain makes the computer "useable" again for me.