At version 9.3.1 access to Microsoft Virtual Earth services is included for ArcGIS Desktop users that are current on maintenance; however, you will need to request a new license file to take advantage of this functionality. If you had registered for Virtual Earth services previously at 9.3, this registration is no longer valid. At 9.3.1, you may not see your Microsoft Virtual Earth map services until you’ve updated your registration file. For more detailed information see KB article 36690.
How to register Microsoft Virtual Earth for ArcGIS Desktop (ArcView and ArcEditor Single Use License) at 9.3.1
For more information see KB article 36696.
How to register Microsoft Virtual Earth for ArcGIS Desktop (Concurrent Use) at 9.3.1
For more information see KB article 36697.
How to register Microsoft Virtual Earth for ArcGIS Engine 9.3.1
For more information see KB article 36698.
For general information on what services are included with Microsoft Virtual Earth and how to use them, please see:
http://www.esri.com/software/arcgis/arcgisonline/virtual-earth-maps.html
http://resources.esri.com/help/9.3/arcgisonline/about/Content/premium_virtual_earth.htm
-Mike H., User Advocacy Group, ESRI Support Services & Sarah O., ArcGIS Online Team, ESRI Development