Presently, it is not possible show Bing traffic data using current version of the RadMap, because it doesn't support multiple map tile providers. But version 2011.Q1 will include this feature, so it will be possible to create map provider for traffic data and show it over the map tiles.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!