They are certainly trying to make it as secure as possible. It is open source, they've held at least one contest to try to break the security of it (http://code.google.com/contests/nativeclient-security/faq.ht...), it is running inside a sandbox with limited permissions, etc.

While it is always risky to introduce anything new, that is a price we pay for progress. And that is true for anything - at this point, I don't think using it would be too much more risky than using a new javascript engine, for example. Aside from never trying anything new, what else can we hope for?

Also, I will point out that even in the dev channel it is still disabled by default. So there is some time yet for the security to be evaluated, to see if there are any endemic problems.