Cloud shadows on a landscape

[George Jenner]

Greetings

Well after the success of last weekend's ploughing I thought you needed 
another challenge :-)

I have a sky coloured using Tim Borgmann's VSL cloud from Realsoft 
downloads.  Now I want to use the same material to put shadows of clouds 
on the landscape. 

It's easy to make a slide projector using a bitmap (it's in the manual - 
light properties, illumination = texture (map coords - parallel map to a 
distant light), but I can't seem to get anywhere using a procedural shader.

So what are my options?  Render the sky to a bitmap and use that?  I'd 
rather use the sky material to easily change the density of clouds in 
the sky and shadows at the same time.

I thought of making a "mask" to put in front of the light, but how do 
you do that for a distant light?  Doesn't make sense for a light that is 
infinitely wide.  Neither does mapping a bitmap to it, for that matter.

And ideas?  This must have been done before.  If anyone remembers links 
I'd be mucho grateful.

Thanks.  But now I have to go thermally morph some disk projections of 
lactofarinacious colloid for lunch. (pancake time)
George