There have been a couple of instances where Sg have created a banner which overlays the top of the Shotgun web interface when a user logs in. These banners inform the user of new features such as 'a new version of SG is coming', or 'get the new iphone app'.
It would be super awesome if we could create our own banners, optionally with a start/end time that we can apply to the site when a user logs in.
For example, if we're know there is going to be down time due to scheduled maintenance then we could let users know in an "in your face" way that they cannot avoid. Emails also work to send out information, but with the deluge of mail that many users get, notification emails can slip through the cracks.