Devang and I wrote up a description of this new LLVM 2.7 feature here:
Extensible Metadata in LLVM IR - The LLVM Project Blog
That's useful but it would be great if you could include programming
idioms/examples showing to attach metadata and how to query it later. For
example, it wasn't obvious to me that you can't attach an MDString to an
instruction but should wrap it in an MDNode first...