Since DNG is based on the TIFF/EP format, it is possible to get the windows explorer to display the embedded thumbnails of all your DNG files, if you do the correct changes in the Registry.
Shaun Ivory, a Microsoft Software Design Engineer explained the procedure for Nikon .nef files in this post at RobGalbraith's forum. Thomas Knoll, the man behind Photoshop, Camera Raw and the DNG format added that you can do the very same procedure with the HKEY_CLASSES_ROOT\.dng entry of your Registry to get thumbnail support right in Explorer!
Remember to ALWAYS back up your registry before changing any value, and that you do it at your own risk... Please also read carefully Shaun's disclaimer.