If the metaphor is that we're looking at the object from above, then the angle of the shadow would depend on the time of day, the day of the year, and the object's latitude. Now THAT would be a neat plugin.
Others have mentioned that Google does it like this in Google Maps...which is even stranger because there is a very clear real world analog in that case. I would have expected that they would take the care to render the shadows as they would actually appear for that location at that time.