This is a great idea, and I agree NaCl is the most exciting thing I've seen in a while. However, the modern JS engines already have JIT compilers for JS. As the

This is a great idea, and I agree NaCl is the most exciting thing I've seen in a while. However, the modern JS engines already have JIT compilers for JS. As these improve more, JS effectively becomes your "bytecode". That's also an entirely reasonable direction that maintains backward compatibility.