NaCL doesn't run "arbitrary" native code; it runs a strictly controlled subset of x86, with all bblock edges verified, in a per-process environment that uses segment access controls to prevent code from overwriting code, and without access to the full system call table. It is basically a faster version of the Java applet sandbox.
Maybe "arbitrary" wasn't the right word; my point is that it looks like BrowserPlus lets you run a small selection of native things approved by Yahoo, while NaCl lets you run whatever you want in a sandbox. Most of the restrictions of NaCl are inconsequential since they only block things that well-behaved code doesn't do anyway.
I believe BrowserPlus will allow 3rd party plugins, though I'm not sure if they will require Yahoo approval.
Also, there's already a "RubyInterpreter" plugin, though assuming it's sandboxed there's not much advantage over JavaScript, unless you prefer Ruby (JS appears to be faster than Ruby, in most cases: http://shootout.alioth.debian.org/debian/benchmark.php?test=...)
