The ArcGIS Online services in the Google
Maps/Bing Maps tiling scheme in Web Mercator Auxiliary Sphere (WKID 102100) are updated regularly and are intended for you to use in place of the services in the older ArcGIS Online tiling scheme. For example, the World Imagery (World_Imagery) map service should be used instead of the older USA Prime Imagery (I3_Imagery_Prime_World_2D and I3_Imagery_Prime_World) services. World
Imagery replaced the USA Prime Imagery service in 2009 and is more up to date.
Online Services Support Status [PDF] provides
information about the levels of support and the current status of different
services. It also includes answers to some frequently asked questions including
how to find out which services you are currently using, and which services you
should switch to if you are still using the older services.