ArcGIS Online Maps Migrated to Google Maps/Bing Maps Tiling Scheme
Good news! Looks like ESRI has finished the migration of ArcGIS Online Maps to a Google Maps/Bing Maps tiling scheme. According to ESRI:
ArcGIS Online map services have been migrated to the Google Maps/Bing Maps tiling scheme in Web Mercator Auxiliary Sphere. This enables Web developers to more easily combine ArcGIS Online map services with other popular Web map services including those from Bing Maps, Google Maps, and other providers using the same Web map tiling scheme.
The existing services in the ArcGIS Online tiling scheme will remain available for at least the next six months and, depending on demand, may remain available longer. Although the services will remain available, the content will no longer be updated.
For more details, see Migrating map tiling schemes in the ArcGIS Online Help.