The next time it wont wake, before going fot the power button, try unplugging the wired keyboard from the USB port for a second and then plug it back in.
That is what I have resorted to, and in my case it works. Easier on the computer than the power button, but still not the greates.
I think my computer may have a problem with power to the USB ports. I had pretty much figured out one was not working properly. Then another one started having problems, and the first "bad" one is now being used.
My IT person suggested getting a powered USB hub, but the _powered hub_ would not worked when plugged into my computer.
When the computer is sleeping, I believe it will only wake with the keyboard. At least that's how it seems with my set up.
Interestingly enough, a coworker of mine, also on an iMac, had a problem where her keyboard stopped working and she had to unplug/replug it in.
Anyway, I would be curious to know whether unplugging the keyboard and replugging it in works for you...
Good luck.