For Bing Maps:
You could easily use the Traffic REST service to provide information about traffic on your control but in fact, there might be issues regarding the Terms of Use as it might be not authorized to use it in another scenario than in a Webpage (whatever the protocol). See 8.2.i (see also point 9 about Mobile APP):
http://www.microsoft.com/maps/product/terms.html
In this case, you'd be in competition against the GPS devices (Garmin and so on) and that's probably the main reason you might have problems to do so. You can reach maplic@microsoft.com if you have a specific use case in mind and want to make sure you would be able to create the application in the way you want.
Anyway, if it's for a Proof Of Concept kind of application, you can use the REST service to get information along a route or for a specific area, see the MSDN here:
http://msdn.microsoft.com/en-us/library/hh441728.aspx
And here is an example to use it in a .Net application (so in your WP client application):
http://msdn.microsoft.com/en-us/library/jj819168.aspx
For Nokia Maps:
Nokia's offer integrates a dedicated traffic API that is not part of the public API but rather from their Nokia Location Platform (also called Here Platform for Enterprise) and with this API you can retrieve information for a specific area and get precise information like in Bing's REST service.
You should reach your Nokia's representatives to get more information if needed about this service.
Let us know if you have any further question.