No release schedule has yet to be publicized by Apple concerning the LLVM 3.1 release date, other than saying May is the likely month, but Bill Wendling of Apple has issued a call-for-testing in preparation for the LLVM 3.1 release. On the mailing list, "Now is the time to start thinking about the Clang/LLVM 3.1 release. Our testers are great people with the patience of Job, but we need more people testing LLVM on more platforms. If you are willing to be a tester for LLVM's 3.1 release, please email me and let me know. The 3.1 release doesn't have a schedule just yet, but testing will be throughout the month of April, with a release date sometime in May."
The officially supported LLVM platforms will continue to be Intel on Darwin (Mac OS X), Debian Linux, Ubuntu Linux, Windows, and FreeBSD. The LLVM developers at Apple are also toying with the idea of releasing ARM LLVM binaries officially for the first time.
A May release of LLVM/Clang 3.1 will just put it about one month after the availability of GCC 4.7; read the prominent changes of GCC 4.7.
Among the features of LLVM 3.1 will be improved AVX support and initial AVX2 support, a.k.a. improved support for Intel Sandy/Ivy Bridge processors and initial support for Intel's 2013 Haswell platform. Other LLVM and Clang 3.1 changes will be talked about in a future Phoronix article.