Hello,
That should be the same ulimit that is set elsewhere, like in:
/etc/init.d/httpd
check the httpd boot script to see if it's there as well, and if you get the same error.
I wasn't able to see any ulimit warnings with httpd (don't have a php-fpm debian box to test)... so not too sure if this is specific to just this box, or others too..
John