Portable Native Client comes down to compiling Google Native Client applications to a subset of LLVM bitcode that can then basically run anywhere that Native Client is supported.
Google's Native Client is used for running native code through their Chrome web-browser in a secure manner on x86 and ARM processors. For making the compiled code portable across NaCl systems, Google has been working on Portable NaCl.
Google has laid out the PNaCl Bitcode as a subset of the LLVM intermediate representation that is architecture-agnostic and just enough to handle all of their needs. This is just another area where LLVM is great and versatile.
The initial Google Portable Native Client Bitcode ABI that was just made public can be found on Chromium.org while some design documents are on Google Docs. Google's Eli Bendersky shared this news with LLVM developers on Tuesday via llvmdev.