3D Skybox (also known as portal_skies in other games)

I was wondering if Unity is able to use 3D skyboxes? I know that Unity can use regular cubemaps to create skyboxes, but I am interested in something else.

If you’ve ever played a game like Team Fortress 2 you’d notice that the backgrounds and distant scenery in the maps are rendered elsewhere and then projected onto the sky. Is such a thing possible with Unity?

What most games do is that they create a sphere around the whole level and texture that, if you do it good it is pretty awesome.
If you have autodesk 3ds max you should have used it once…