For the WebMercator case and images that cover a small area, this has been fixed in the ArcGIS for WPF version 10.1.1 and ArcGIS for Silverlight version 3.1.
For others map spatial references, it's not fixed. The workaround may be to project the groundoverlay extent by yourself by using a geometry service. This may make sense for SpatialReferences whose the projection of a WGS84 envelope is also an envelope (i.e a projection where meridians and parallels are perpendicular straight lines).
But anyway this will not be accurate for images that cover a large area (due to the image distorsion)