Hdr world lightning plus hard shadows?

I’m creating architecture scene using world angmap for lightning. It’s looking nice, but i want a tree cast shadow on my building. If i use sun lamp and render it - it illuminates everything and cast tree shadow, but i need only shadow and if i try and check “only shadow” box, it didn’t cast anything at all. Blender 2.57

Try putting the sun and tree and a duplicate of the building on a layer all by itself. Then make the building ‘Shadows and Distance’ only.
The sun should have something like ‘only this layer’. Just going from memory here, but that should get you started.