Hi,
In the current ArcGIS Runtime SDK for WPF Graphic objects must be created on the UI thread. However, you could try creating a collection of geometries (e.g. List<Polygon>) on a separate thread and return that for the Graphic creation on the UI thread. This may provide some benefit if you are constructing polygons (the difference is negligible for MapPoints).
In the new ArcGIS Runtime SDK for .NET (currently in Beta) we have clarified the threading model so anything which is related to the Map must be created on the UI thread (i.e. Map, Layers, Renderers, Symbols) and content which is already part of a Layer such a Graphics. However, it is possible to create Graphic objects on a separate thread and then add those to a Layer on the UI thread. If your release schedule is beyond summer 2014 you should look at the new .NET SDK.
Cheers
Mike