Mantle was announced last year as a closer-to-hardware graphics API to complement OpenGL and Direct3D. Mantle is designed to reduce driver overhead and deliver better gaming performance. To date the Catalyst Windows driver supports Mantle, there aren't any other vendors/drivers supporting Mantle, and there's only a handful of games currently shipping with a Mantle renderer.
I asked several AMD people this week about the Mantle prospects for Linux, given that it's been said before they would like to see it on Linux. The common response I received came down to that they're currently looking at the feasibility of Mantle on Linux and that it comes down to a resource issue in committing to any Mantle Linux support plans. Right now they're not yet ready to announce anything but are investigating and don't yet have any Mantle Linux code.
Of course, for Linux game developers looking to maximize the performance of OpenGL instead of switching to a totally different API, checkout the talk about reducing OpenGL driver overhead. If you also missed the Saturday article, there's the Phoronix exclusive on AMD Is Exploring A Very Interesting, More-Open Linux Driver Strategy.