Someone made the point when the new iMac was introduced that any cable used to recharge a wireless keyboard or mouse would be left plugged in by most people 90% of the time, completely negating the point of them being wireless. The logitech wireless mouse I used with my old B&W G3 tower was nice, but stubbornly insisted on needing the battery changed at 3am, in the middle of one game or another. Most annoying.
Any sort of charging cradle would simply add to desktop clutter. Also, with an iMac especially, the screen is too small to be usable from any distance further than the keyboard cable already allows... and as for the mouse cord, I use trackballs.. so that's a non-issue as well.
As a final point, wireless would kinda put the kibash on having USB ports on the keyboard.. I don't think anyone makes a wireless keyboard (or a technology to power a wireless keyboard in it's current size) that would support a USB hub and have a battery life that would be acceptable. I could be completely wrong on that, but the constant communication that a USB hub would require would be levels of magnitude greater than a simple keyboard requires. For all the talk about USB being hot pluggable, it's great at causing kernel panics if you unplug something when the OS isn't expecting it, so there would be no sleep or reduced power for the keyboard just because you weren't typing.. at least not if you had a USB device plugged into it... which would just be another tether to drag around to anywhere you took your "wireless" keyboard.
So, you would have to axe the USB on the keyboard, add an RF transceiver (and shielding) to the iMac, add an RF transceiver to the mouse and keyboard, and some way to power both of them. Oh, and don't forget you have to make a totally different model for schools, since both the mouse and keyboard would walk on a daily basis. And get intentionally switched between units on an hourly basis. Still sound like a good plan?