Since the end of January 2011:
"ArcGIS Online map services are available at no cost regardless of use." - http://www.esri.com/software/arcgis/arcgisonline/standard-maps.html
However, the use of ArcGIS Web Mapping APIs may require a subscription. - http://www.esri.com/software/arcgis/arcgisonline/common-questions.html
All the basemaps available from ESRI are accessible by REST services which do not require an API. There is already a new layer type in OpenLayers that can access these services.
The Terms of Service state:
In particular, you may not (or attempt to) (a) access Services or resources not made available to you through a standard web browser or Esri products;
Update - these TOS may in fact refer to the esri.com site and "services" rather than the map services themselves.
So is it legal and within the terms of service to use any of these layers in any web application such as OpenLayers or through the Google or Bing mapping APIs as these are all used in a standard web browser?