In IIS6, I notice that when "Enable content expiration" is not enabled, no cache-related or expiration headers are sent with the response. Yet static content, such as css files are properly returning 304 statuses on subsequent visits.
When does this content expire if nothing is specified? Is it browser dependent?
If Content Expiration isn't specified in IIS then the caching of website content will be controlled or affected by external factors such as your browser's settings, upstream proxies or content caching servers between the end user and your web server. These are often factors out side of your control.
Setting content expiration in IIS should force these 3rd parties to honour these caching settings and expire content at the correct time.