Transfering a Bluetooth keyboard & mouse from one iMac to an iMac that has never used Bluetooth Is there any way to do this? I tried putting iMac 1 to sleep and waiting for the 2nd one to pick up the keyboard and mouse, but still "No keyboards have been found."

The 2nd one is, of course, the one I need to use with a few hours. I tried downloading certain programs listed in other answers, but they did not pick up iMac 2.
You'll also likely need to turn your keyboard off/back on and make it discoverable.

See:

http://support.apple.com/kb/ht1809

for a complete walk-through, but to be safe, turn off first mac, off keyboard, then back on while searching on the second Mac (using bluetooth control panel)

Alternately, instead of fully turning off the first Mac, you could just turn off its Bluetooth, which is much faster. But make sure you have a way to turn Bluetooth back on again—if you only have Bluetooth peripherals, this could be problematic.