Are you saying it's possible for me to use my 2011 27" iMac as a screen for my Xbox 360 or my 2009 Macbook?
No. There seems to be a misconception of what Thunderbolt is. Thunderbolt is an extension of the PCIe bus, allowing external peripherals to operate as though they are connected to a computer internally. Thunderbolt is not natively a video protocol, but it allows Displayport data to be multiplexed into the Thunderbolt data stream. At present Thunderbolt only carries a 4x PCIe signal, but if it reaches the full 16x, it would be possible to have an external card cage with multiple nVidia GTX 680 desktop video cards in SLI mode, connected to an iMac for gaming, as iMacs have desktop CPUs.
The Thunderbolt display and iMac have minidisplayport output, but not input. That's an Apple decision. If they accepted minidisplayport input, then any device with minidisplayport output could be connected using a minidisplayport cable.