You can customize these areas using Ribbon extensibility, and supply Ribbon XML to the GetCustomUI method of the IRibbonExtensibility interface. Use the Ribbon ID for the feature to determine the XML to pass as a returned value in the GetCustomUI method. Your Ribbon XML typically supplies callbacks that let you respond to a button click, or control visibility or the icon of your Ribbon controls. Use the IRibbonControl.Context object to identify the active window containing the user interface that triggers the callback procedure.
Documentation for the Solutions module and customizing Outlook user interface is currently available to participants of the Technical Preview program. Most developer documentation for Outlook 2010 new features will be available to the public before the launch of Office 2010. I will keep you all posted about further documentation availability.
See also on MAPI http://msdn.microsoft.com/en-us/library/cc765775(office.14).aspx