Add this as a reference if someone else want to use SketchFlow.
Geometries?
To illustrate geometries I use, Pen for polylines:

To get a sketchy look I change the Stroke by choosing BaseBorder-Sketch.

Rectangle-Sketch to fast draw polygons as rectangles :) otherwise it should be possible to use the Pen tool for this as well:

Circle-Sketch for points:

Here I change the background and border to BaseBorder-Sketch to get a filled point.
Resulting in something like this:

Icons?
For icons on toolbars and other image resources I found a note on Import current UI into sketchflow about using Paint.Net to create sketchy looking images.
To do this open an image in Paint.Net and use Effects | Artistic | pencil Sketch...

Then add the images as background in buttons to create a toolbar or just add the image to the screen.

ArcGIS Silverlight/WPF Web APIs
I've also looked at the possibility's to use the ArcGIS Silverlight/WPF web APIs to do this but it felt like it's a bit complex to easily add concept to a prototype.
But if you want to add some background map. It can be done by adding the Map control that gets integrated in SketchFlow when installing the web-API:

Add layers by using the properties:



It's also possible to add a GraphicsLayer to add a Polyline using the XAML editor:

Well that my findings so far...