LLVM/Clang for a few releases has supported C++11 but the compiler code itself has been written in C++98. However, it was agreed upon following the most recent LLVM/Clang 3.4 release that the compiler developers themselves would be free to rely upon C++11 features.
LLVM developers for a few times have been after using C++11 functionality but now for the LLVM 3.5 release cycle they have finally agreed to allow it now that C++11 support is widespread enough upstream for supporting it as the host compiler. This C++11 usage is just not for LLVM itself but also Clang and other LLVM sub-projects where they're free to write C++11 code, albeit it's a targeted subset of C++11 for broad compatibility.
The new host compiler requirements for LLVM going forward are slated to be Microsoft Visual Studio 2012, Clang 3.1, and GCC 4.7.
When the build systems switched over on Friday to compiling in C++11 mode, nothing appeared to break and over the weekend Chandler Carruth will begin removing support for building in C++98 mode. These details were shared Friday night on the LLVM mailing list. The new C++11 coding standards being evaluated by LLVM developers are documented on this web-page.