John Carmack says about it:
"Considering the boost Mantle could give to a steambox, MS and Sony may wind up being downright hostile to it," he said. "I don't know the details, but it is pitched as a console level hardware access for the PC from AMD."
There is more info from APU conference:
1) Mantle will be standalone API.
2) It put responsibility of preparing command streams, memory management, mulit gpu utilization (deciding what work gets done where), etc. on application.
3) Its just great
Will see if GLSL is supported by Mantle.
And I'm not so sure Gallium is suited for implementing something like Mantle. But who knows? Or maybe AMD this time do the Mantle driver the right way, and they will be able to opensource all or most of Mantle?