Can I use my Mac keyboard as input to another Apple device? Is it possible to use the native keyboard on one Mac, to control the keyboard input of another Apple device, especially another Mac? 

And so doing, without having to completely redirect the input device, i.e. disconnecting/connecting a Bluetooth keyboard to switch it between devices.

In my setup, I have a MacBook Pro and an iMac. Ideally, on the MBP, I would like to press a keyboard command or an GUI icon to connect the built-in keyboard as input for the iMac.

Note: Screen sharing is not a viable answer for this question.
1Keyboard is an OS X app that allows a Macintosh keyboard to type on another Mac, iOS devices like iPhone and iPad, Apple TV, PS3 and Android.
I followed the pairing instructions, and while it didn't work the first few tries, it does now. Once I got it up and running, it has worked just fine. Copying, Pasting, keyboard shortcuts and Media keys (Volume, Playback) are supported.
You can set keyboard commands to connect to different devices. I use Shift + ⌘ + &gt; to connect to my iMac, Shift + Ctrl + &gt; to connect to my iPhone, Shift + Alt + &gt; to connect to my iPad.