btw i have done some research and found some really nice article about deffered lightning. but none about shadowing which is kinda making me believe that its not something u should do or maybe no one has simply done...or i'm to stupid to find it. anyway keep up the good work.
Yes I was almost about to say what you suggested was impossible, because you would need to see the scene from the cameras point of view with the shadow map projected from the light source's point of view for each light, because the whole point of shadowmapping is to shadow those points that have the wrong depth from your point of view based on the light source's projection.
But then I got to thinking, and I realized there definitely is a way. If you render the scene depth from the camera's point of view, you can interpolate between the 4 normals of the screenquad (That are pointing out into the scene) based on the screen's coordinates and shoot a ray into the scene, if you multiply the normalized direction of the ray by the screen space depth, tada!, you get the world position. This little trick was used by the crysis guys for things like SSAO etc (Don't think they used it for the shadows however.).
Anyway, we now have the world space pos, but how do we sample the projected shadow map texture at that position? Simple, just multiply that world space pos by the respective view and projections matrices (In the right order!), divide by W and scale, and hopefully, you will get the correct screen coordinates for the projected shadow map for that particular screen pixel!
This would be quite useful if you had alot of lights with highly tessalated geometry, it would avoid having to render them again and again. I think it would also avoid a few state changes like render target switches, etc, if you do it right.
One problem might be the matrix multiply per pixel, not sure how expensive this is but it might raise the instruction count a bit. I think for most practical situations however with multiple lights this technique will outperform the standard direct rendering.