Creating a depthmap out of two stereoscopic 3D pictures is possible with software like Stereo Works 1.0. It is avaiable at http://www.lycosoftware.com/SW/Sw.html#Download for free. The results are quite good - even they are still not perfect.
As well Triaxes (r) offers with Stereo Tracer “Professional” a Stereo and 2D plus Depth to 3D Converter. This software is designed for rendering multi-view images out of one picture and a depth map and can create a disparity image or Z-image out of two stereoscopic images. The software might be the best on the market at this moment, but the depthimages are still not perfect. Demo is for free and worth a try. Automatic rendering a depth map from a stereo pair to download from Triaxes.com : http://www.triaxes.com/index.php?id=59
This might be interesting for creating disparity images too: http://www.shawnlankton.com/2008/04/stereo-vision-update-with-new-code/