Sites that do user notices on a regular basis often create a system where small bits of text go into a DB along with start & end times for display, the admin who posted it (for auditing purposes), countries where it's visible, etc. There's an ACL for who's authorized to post new notices. Then the main site template has a section that's shown only when there's an active notice and automatically disappears when the notice is stale.
All this is probably overkill for a site like HN. However, think about things like tsunami warnings on the Google homepage. These get shown only in a certain geographical location, they need to be approved by multiple people, and they need to get pushed out to tens of thousands of servers, all within minutes of the alert coming in. There's a lot of complexity that can go into even a simple feature.
