How do I set a recognized Apache DocumentRoot under Lion?
Things do doublecheck:

1. File permissions

The specified path for the new document root should allow at least read access for the Apache user, which is _www by default. You probably want to chmod 755 /Users/jonathan/mirror if the file permissions are too restrictive. What are the actual file permissions on the directory and files?

2. Virtual hosts

When you enable name based virtual hosts, the document root in the main config is ignored. Instead the root for the matching hostname will be used, and if none match it will default to the first virtual host.