Because of a recent addition to the the Gentoo PHP package (FPM support) they're looking for people to test it out and report back the results. To help, Ole Markus has written up some instructions on how to get it up and working on your system with nginx.
PHP just added the FPM patch to their 5.3 branch, making it likely that PHP 5.3.3 will support the FPM SAPI. The FPM SAPI is an improved fcgi SAPI that allows for more advanced configuration than the original fcgi SAPI. One of the most useful features is process management, which makes it very useful for lightweight webservers, such as nginx, that does not handle process management of fcgi themselves.
There's only a few commands you'll need to get this new package installed and working with your local nginx install with the help of the emerge and layman tools. He also includes the configuration change you'll need to make to the nginx config file to get it working as a FastCGI module.