Like you, I'm doubtful about the true openness of this. Proof of the pudding, in my book at least, will come when some group of hobbyists take it up.
True, but then again there's a world of difference between "works best with AMD" and "works only with AMD" - best outcome for AMD would of course be the former. I'm sure they've learned the lesson of how badly NVidia botched PhysX.
I'd heard that NVidia were "considering" it based on opinion expressed by folks like Eidos etc. And I've also read that the publishers are definitely "pro" because Mantle support is/will-be available for Windows and Linux/SteamOS, (and perhaps even consoles I guess), because that's going to make their porting efforts a LOT easier.
Intel on the other hand, I really could care less about. Sure, their IGP's power a lot of laptops and low end desktops, but surely they're a distant third behind teams red and green?