I tried pretty much all of the approaches outlined above without success on nginx 0.6.37 (posting this in June 2009. Get the version # using: nginx -v) The one approach I found did work was putting the following in the server section of my config files:

error_page 503 @503; location @503 { rewrite ^(.*)$ /system/maintenance.html break; }

if (-f $document_root/system/maintenance.html) { return 503; }

I found it in the internet archive, posted by Lee Jensen and credited to Ezra.

K.