Hello Lou,
It seems that the problem is in the custom provider, or in the way you use it (do you limit geographical region for custom provider using GeoBounds or GeoBoundsNW and GeoBoundsSE properties?). Everything else seems to work just fine. You can check it in our demo application:
http://demos.telerik.com/silverlight/#Map/TrafficProvider (this sample uses Bing traffic and OSM providers).
http://demos.telerik.com/silverlight/#Map/MultipleTileLayers (this sample shows geographically limited OSM over the Bing)
Or you can use following XAML (it show semitransparent OSM over Bing):
Best regards,
Andrey Murzov
the Telerik team
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