While the Q1-2012 FreeBSD Status Report was just talked about in the afore-linked article, the Clang and C++ items warranted their own more detailed article.
Going back to 2009, FreeBSD developers have worked to replace GCC with LLVM/Clang. Over time, there's been numerous improvements especially with the FreeBSD 9.0 release. The FreeBSD developers are interested in doing away with the GPL-licensed GNU Compiler Collection and instead use the Apple-sponsored LLVM/Clang work that's under a BSD-like license.
As reported in the status report for the last quarter, Clang 3.0 is now installed by default for FreeBSD 10.0-CURRENT and 9.0-STABLE. They're also working to import a newer Clang snapshot since LLVM 3.1 and Clang 3.1 are about to be released (next week). On FreeBSD 10.0-CURRENT, Clang can now build world and the generic FreeBSD kernel without emitting any error warnings. With more recent revisions, a new WITH_CLANG_IS_CC option will make Clang become the default cc, c++, and cpp commands.
The intent is to make Clang become the default compiler for FreeBSD 10.0 and to then deprecate GCC from FreeBSD. "The intent is to switch on this option by default rather sooner than later, so we can start preparing for shipping 10.0-RELEASE with Clang as as the default system compiler, and deprecating gcc."
Additional information on building FreeBSD with Clang is available from this FreeBSD.org Wiki page. Any FreeBSD package not building under Clang should be treated as a bug; "One of the most important tasks at the moment is to actually build and run your entire FreeBSD system with Clang, as much as possible. Any compile-time or run-time problems should be reported to the appropriate mailing list, or filed as a PR. If you have patches and/or workarounds, that would be even better."
There's also been other non-Apple operating systems toying with Clang for different use-cases. There was also the Debian Clang experiment, which could build most of the Debian packages for popular architectures, but LLVM/Clang still falls behind in its support of the more obscure CPU architectures and platforms compared to GCC. Some vendors have also been playing around with the idea of using Clang to build the Linux kernel (it's possible to do with certain kernel configurations, patches, and other headaches).
LLVM/Clang is unlikely to replace GCC on any tier-one Linux distribution as the default in the foreseeable future, but it's interesting as a complementary option. Clang is quite good with its static analysis abilities, among other benefits.
In terms of FreeBSD's mission for a GNU-free C++11 stack, the libc++ and libcxxrt libraries have gained new features, support for the ARM EABI, and with Clang 3.1 can now pass all C++11 atomic tests. The libcxxrt library is the new C++ run-time that was given by PathScale. There's still some work ahead in this area, but the status report mentions, "we're on track for a BSD licensed C++ stack in 10.0."