http://integrate.onterrasys.com/BingMapsWMSBeta/default.aspx
"This is a beta service is provided by the experts at OnTerra Systems.
It allows you to access Bing Maps base data such as ortho imagery,
road data and hybrid layers (road + ortho) in Open Geospatial
Consortium (OCG) - Web Mapping Service (WMS) format. This service
works in conjunction with any WMS capable application. Currently this
beta service supports WMS 1.3.0 and 1.1.1 standards, and primarily
using web Mercator projection. "
Diego Guidi