Hey, this is cool:
This is the first I've seen it disclosed publicly, but the summary is that Google plans to make Native Client (NaCl) portable with some kind of verifier on LLVM bitcode. Now, if you're familiar with LLVM, then you may have heard the claim that LLVM bitcode from a C compiler is not platform neutral. The argument is that you always have things like #ifdefs that check for the target architecture and that most C compilers (maybe including Clang? I'm not sure) lower things like sizeof(int) in the frontend. Therefore, they claim, it is essentially impossible to compile C and C++ down to a portable representation.
However, it seems doable for NaCl, because you're not dealing with arbitrary programs. In their writeup, they explicitly say you should eliminate things like architectural #ifdefs. They also state up front that sizeof(int) == sizeof(long) == sizeof(void*) == 4. I'm not sure how that fits in with x86_64. They also state that the data model assumes that integers are little endian, which isn't exactly portable either.
Still, it's a cool project.