Documentation for using bing Maps can be found at:
http://help.arcgis.com/en/webapps/flexviewer/help/Default.htm#CSHID=layer_tag.htm|StartTopic=layer_t...
help for using Bing in LocateWidget can be found here:
http://help.arcgis.com/en/webapps/flexviewer/help/Default.htm#CSHID=widgets%2Flocate_widget.htm|Star...
See it in action at:
http://help.arcgis.com/en/webapps/flexviewer/live/index.html?config=../samples/config-bing.xml
config file for the above application is at:
http://help.arcgis.com/en/webapps/flexviewer/samples/config-bing.xml
Config file for the Locate Widget used in above application is at:
http://help.arcgis.com/en/webapps/flexviewer/live/widgets/Locate/LocateWidget_Bing.xml
More info on getting Bing key can be found at:
http://help.arcgis.com/en/webapi/flex/help/index.html#ve_gettingstarted.htm#