Brought to my attention by @serbrech
If the Source of an Image is null, the page that contains the image will not be printed (fails silently).
This happens when the runtime is Silverlight 5 (Vector printing). When the runtime is Silverlight 4, the page are rasterized as bitmap and all is fine.
Compiling with SL4 or SL5 RC tools produce the same behaviour.
A possible workaround is to wrap the Image control into another control that will swap a null Image source with a blank image.
I may make some changes to the sample in response to this.