Over the weekend the ArcGIS Online basemap galleries found in the ArcGIS.com map viewer and ArcGIS Explorer Online were updated. The Ocean and Light Gray Canvas basemaps now replace the Terrain and USA Topo basemaps.
The changes were made since the Terrain basemap is already included in the Terrain with Labels basemap, and so was redundant; the USA Topo basemap was replaced in favor of basemaps having more global coverage.
Both the Terrain basemap and USA Topo basemap will continue to be available on ArcGIS Online, and can be discovered via search or by going directly to the shared items:
Terrain basemap (with reference layer)
USA Topo (as layer on top of World Topographic basemap)