According to nginx modules list in $URL: Limit Zone Limit simultaneous connections from a client. Deprecated in 1.1.8, use Limit Conn Instead. Limit Conn Limit concurrent connections based on a variable. While upgrading to nginx-1.2.1 (current stable version) I can't emerge with "Limit Conn" module.
Thanks for the report, Builds successful with 's/limit_zone/limit_conn/' - will be implemented on a version bump since development is pretty fast.
fixed in 1.2.2/1.3.3