The Native Client paper[1] was published in 2009. The main reason it took so long to be officially released is that it underwent a lot of scrutiny by various security researchers[2], through the Google bounty program. In fact, NaCl has been shipping with Chrome all along; it just needed a flag to enable it (--enable-nacl).

You should read their research paper for interesting insights.

[1] http://www.chromium.org/nativeclient/reference/research-pape...

[2] http://code.google.com/contests/nativeclient-security/