Among the improvements to be found out of LLVM 3.3 include better Intel Haswell support (improved AVX2, etc), some noted performance improvements, Clang will have better C++11 support, the long-awaited AMD R600 GPU LLVM back-end, 64-bit ARM / AArch64 support, and many other features to be discussed in the coming weeks.
Bill Wendling's release announcement can be found on the cfe-dev list. Basically, testing will happen in May and a release will come in June. Information on branching of the code-bases and the release candidates will come later.
Right now they're planning for official releases on Mac OS X x86, Ubuntu Linux x86, FreeBSD x86, and experimental Windows support. LLVM developers also want feedback on other operating systems and CPU architectures to officially support with LLVM 3.3.