This seems like more of a ChromeOS move to me, so that you can run native code on Atom and X86 machines. The knock on ChromeOS is always that it's "just a browser". I seriously doubt anyone is going to write their Etsy storefront in NaCl. The benefit here, of course, is that if someone does write the next Photoshop for ChromeOS machines, it should also run fine in OS X, Windows and Linux. Seems like a win for portability to me.
PNaCL and asm.js are roughly the same in terms of "exposing" the code in this sense. Both can be easily disassembled/prettified using readily tools readily available today.
I doubt Google wants to keep this Chrome-only. Just like SPDY, there isn't initially support on all platforms/browsers so you have to have a fallback. But if it catches on then you will see broader support over time.
Having taken a good hard look at adding this to a non-chrome browser I can tell you it would not be a trivial task. You can't just take the code and re-use it elsewhere and re-inventing it would require constant catch up.
You can't adopt the implementation of PNaCL either. It depends on PPAPI, which itself depends on vast amounts of implementation details of Chromium. There's no real attempt to decouple them.
People have in fact looked at what it would take to adopt PPAPI. The answer so far seems to be "you have to use Chromium, not another rendering engine".
The big difference, of course, is that you can be on any operating system with Chrome/Chromium. It's not ideal but that is huge. The "lockin" is no longer at the operating system.
Can't wait to log into a bank that demands I installed Chrome to use their new 'security' or some web apps that won't run in anything but Chrome.