How can a keyboard be shared with an iPad? What are the ways for both an iMac and an iPad to share the same physical keyboard?

If it's possible to share a regular Apple bluetooth keyboard between both devices without jailbreaking the iPad, I think it would be my favored option.
I don't think you can. Bluetooth is designed such that pairing is two-way (computer &lt;-> device). You can't pair to a second device without un-pairing from the first device.