Nope only be able to use it for physics, not graphics rendering.
R580/580+ and R600 will be so fundamentally different in design as to not work well in CF without serious tweaking to the point of an R600 emulating an R580, which loses most of it's benefits.
It is technically possible to do it, but I doubt it'll ever be supported by ATi as it would cause a ton of driver developement headaches, it could probably be done for a basic 'proof of concept' type of thing but for the general public consider it a no go; kinda the difference between impossible, and impractical/implausible.
Not a chance.
Doubtful, cool if it did though
...and I doubt that ATi would desire to do that, since it would mean an elongated lifespan for the older cards. By not allowing it, it would mean that you have to fork out more money on a new Gfx card. More money in their pocket...
CF not a chance. Because of different kinds off architekture of the GPU's. But You can put one for rendering and one for HavokFX Physics. R5X0 for render R600 HavokFX. Or otherway around.