You might check into ESRI ArcWeb Services -- they host a variety of other commercial data sources (teleatlas, navteq, globexplorer, etc.). They don't have native WMS or WFS (last year I did some work integrating ArcWeb into Mapserver as a means of WMS access). I know ArcWeb has some hillshade imagery, Natl Geographic TOPO! imagery. not sure about additional DEM sources.
hope this helps, REH
