Hi Scott Harrington,
We will have this feature implemented for Sitefinity 4.0. Currently we are working on a tool and most probably it will be available for downloading at http://www.sitefinity.com/marketplace.aspx
in a few weeks.
A simple implementation at this stage could be adding a logic in Application_Start method of Global.asax that checks DateTime.Now and set ICmsPage visible or not using Navigable property.
the Telerik team