In talking with some of the editors of AnandTech, apparently the limiting factor here is how DirectX applications would react if you were to disconnect the external GPU from the device and Microsoft would need to be the one to architect a better system reaction than a blue screen of death. There is one MSI demo showing this working, but it was never released.
So maybe 1-2 years? Heck, we'd need Thunderbolt on more non-Mac computers first.
Intel Core i7 2600K | Radeon HD 7970 | MSI P67A-GD80 | 240G OCZ Vertex 3 | 16GB RAM | Dell U2713HM | Windows 8 Pro
2012 Mac mini | 256GB Samsung 830 + 1TB HDD | 16GB RAM
15" Retina MacBook Pro | iPhone 5 32GB | Nexus 7 (2013)