Unfortunately libc++ is fully supported on Mac OSX only, with ports on their way. I actually use libc++ on linux but there're still some things that need to be fixed. [1]
yeah right. It is beta-quality unfit for pre-packaging in distro's. ABI incompatible with libstdcxx and deliberately OS X only. (It works sure (i mean kinda) no one is actually a linux port developer or maintainer not yet)
Not just Mac OS X, it is in the FreeBSD base system by default, it is the replacement for the libstdc++. See FreeBSD 10 where the switch was made :-).