Also, the packaging is done differently by ubuntu and nginx upstream, so I don't think you can just replace it. They are also bundled with different modules. For example, I have to use the package from ubuntu because the one from nginx upstream lacks the geoip module.
Following up - this fixed it! (Took a while to get a virtualbox set up with my old config and simulate the upgrade a few times to make sure everything was the same)
Nothing wrong with PHP itself, but the stock/newer nginx must read configs differently, and so it's not a simple "remove old nginx, install new nginx" upgrade, as it no longer served up php when I upgraded...
No errors thrown anywhere, but php5-fpm is dead in the water... Rolling back until I can find anything on how to fix this.