What Graphics Cards support Metal?

Nope. You barely missed the cut.

• Nvidia – GeForce gtx 4xx and newer
• Intel – HD4000 and newer (ivy bridge and newer)
• AMD – HD7000 and newer

Where do you have this info from?

Only HD7000 for Radeon ? That's quite restrictive...

Do you know if there will be CPU devices available, just like in OpenCL ?

And multi-devices (for MacPro) ?

Don't know about CPU devices for Metal but the documentation metions a set of API calls to retrieve Metal capable devices.

1. MTLCreateSystemDefaultDevice()
2. MTLCopyAllDevices()

According to the header file, MTLCreateSystemDefaultDevice will return the GPU associated with the screen for multi-gpu system. MTLCopyAllDevices gives you all the capable devices which you can then pick based on what features you want. On my Mac Pro it returns a device for each GPU.

bugger. i really thought that metal would be something good for my mac. is there a way to force it on older hardware