Some additional info here and I think I've clarified that this isn't an nginx codebase issue: After a 'sudo nginx -s quit' and confirming that no more nginx processes were running or local web sites could be accessed, I was still seeing entries to the log file made, both warning me about the user directive and another permission issue:

2011/04/21 11:50:45 [emerg] 34505#0: bind() to 0.0.0.0:80 failed (13: Permission denied)

I wasn't sure what else could be writing to the nginx log file directory, since neither the Activity Monitor nor 'ps -ax | grep nginx' showed any nginx process running.

Then I looked at the org.nginx.plist LaunchAgent that starts up nginx and saw that it had KeepAlive set to true, suggesting it was in charge of restarting nginx. I guess I was mistaken in the ps option I used to find all processes. The relaunch probably happened under a different user ID, generating the errors.

I'll take things up with the author of this plist (which came with brew).