I was one of the first committers on Mongrel2 (actually, second behind Zed) and I still am (though mostly as a historical accident, I haven't contributed anything of substance since SSL).

In my opinion, this is great. I think Mongrel2 is an excellent proof-of-concept, and if it provided the impetus for a much more widely used project like nginx to go down a similar path (though it's just a module right now), then everyone wins.

That said, this certainly isn't in nginx mainline, and it's very feature poor as compared to Mongrel2. One could not simply swap in nginx with this module for an existing Mongrel2 installation.