1 Keyboard/Pointer, Many X Servers?

[Hibiki Kanzaki]

I want to have several video display adapters/monitors
with my computer, each with a corresponding X server,
only one of which is actively receiving keyboard/pointer
input at a given time.  I have one keyboard/pointer
attached to the computer.  What are the simplest and
best ways to configure things so I can switch the
keyboard/pointer from one X server to another?

(note: I do not want to have one X server span multiple
displays, and I am not looking to have a single virtual
desktop which seamlessly spans multiple X servers; I
just want to be able to switch from one to another)