UPDATE : SPDY patch is now part of nginx >= 1.3.15
NGINX Ebuild with SPDY patch
See http://en.gentoo-wiki.com/wiki/Overlay#Local_Overlays to see how to install local overlays
Then clone repository :
git clone http://github.com/paxal/nginx-spdy
Unmask the ebuild : add ebuild to /etc/portage/package.unmask and /etc/portage/package.keywords, and add the nginx_modules_http_spdy use flag.
This package requires >=dev-libs/openssl-1.0.1. This won't break linked binaries as the library is still libssl.so.1.0.0.