In each series, the first image is the final results, the second image is the contribution from the skylight and the third is the direct sunlight illumination:
This is already looking good, and it runs in realtime. If there was some large character moving around in the scene, you would see its contribution to the shadowing.
The skylight is a standard shadow map, but it is sampled taking a lot of neighbors into consideration. This creates nice soft transitions between areas of light and penumbra.
There are no light bounces here, however. I have been toying with the idea of a screen-space radiosity solution. I think it is possible. If you know of any papers on the subject, please drop a link in the comments section.
As usual I look forward to your comments and criticism.