Since the end of 2013 a free use of Bing Maps in ArcGIS is no longer possible. You have to obtain a Bing Maps key directly from Microsoft.
In QGIS (with OpenLayers plugin) it is possible to use Bing Maps (and also Google Maps) without a Bing Maps key and without a fee.
Why it is possible to use Bing Maps with QGIS for free while you have to pay for it with ArcGIS (Desktop)?
ESRI source: http://downloads2.esri.com/ArcGISOnline/docs/bing_faq1.pdf