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What about security? Do they have a layer enforcing the sorts of constraints that javascript is currently restricted by?

Much better security architecture. If you're technically inclined, you should read their paper about the NaCl architecture. http://nativeclient.googlecode.com/svn/trunk/src/native_clie...

Indeed, the point of NaCl is that it's not going to be another ActiveX-style security nightmare, so people might actually use it. That will be great.