CM -- here's how I've been doing my shadows for almost a year now:

1. Say it's a bunch of houses, ctrl+click on the houses layer to load it as a selection.
2. New layer, fill it with black, deselect.
3. Duplicate the layer, move it up and over (or down and over) with the arrow keys on the keyboard 1 up and 1 over.
4. Repeat step 3 as many times as necesaary until I have a nice looking shadow.
5. Merge these shadow layers, put the layer under the houses layer, and lower the opacity of the layer and sometimes a blur.

If I want to to a tower or something with a tall shape then I draw it in profile and then just rotate it to fit the sun angle, then do step 5.