LLVM 2.6 includes better x86_64 code generation, new code generators for multiple architectures, support for SSE 4.2, improved optimizations, and perhaps most notably it is the first release to include Clang where it's now at a "production quality" status for C and Objective-C on x86. Clang is the C compiler front-end for LLVM that replaces the need of a GCC front-end within LLVM. Clang in LLVM 2.6 offers better error messages over that of what GCC produces, compiles code noticeably faster (reportedly a 3x improvement over GCC 4.2), and is BSD licensed.
There are also new LLVM sub-projects, such as DragonEgg, which is a llvm-gcc plug-in now targeting the unreleased GCC 4.5 series. Furthermore, included in LLVM 2.6 is an early version of llvm-mc, which is a machine code assembler and disassembler.
The release announcement for LLVM 2.6 was made on the LLVM mailing list.