Here's a video from the 2010 LLVM Developers' Meeting that introduces libc++ and shows how it builds upon new features introduced in C++11: http://devimages.apple.com/llvm/videos/LibC++.mov (16:36, 42MB)
Just to say, libc++ is already packaged with recent versions of xcode.
One reason I find this particularly exciting is that as Apple funds a lot of the clang and libc++ development, non-OSX support is not a top priority for them (although patches are greatfully received).
I wasn't sure if there would be the motivation to get libc++ running, and keep it running, on other OSes. I assume this move will ensure libc++ keeps moving. One advantage is that there aren't that many OS specific bits, and they tend not to change that much over time.
Slides: http://llvm.org/devmtg/2010-11/Hinnant-libcxx.pdf