NaCL is not portable, unless you happen to be running the same architecture that the software was compiled for. You might be thinking of PNaCL which is still very experimental: http://www.chromium.org/nativeclient/pnacl
Well that's just dumb. They should've made ARM a first citizen for ChromeOS from day one. I saw a rumor about a future Samsung Chromebook that will use a dual core 2 Ghz Cortex A15 chip, so maybe it's coming soon.