Currently, you can achieve this by using RotateTransform and clipping the container that holds the RadMap control. For example:
<Grid RenderTransformOrigin="0.5,0.5" Width="500" Height="500">
<Grid.RenderTransform>
<RotateTransform Angle="45" />
</Grid.RenderTransform>
<Grid.Clip>
<EllipseGeometry Center="250,250" RadiusX="250" RadiusY="250" />
</Grid.Clip>
<telerik:RadMap ZoomLevel="6" Center="37, -120">
<telerik:RadMap.Provider>
<telerik:OpenStreetMapProvider />
</telerik:RadMap.Provider>
</telerik:RadMap>
</Grid>