I am using the DNG codec downloaded from http://labs.adobe.com/wiki/index.php/DNG_Codec a week or so ago. Using the WIC API both the IWICBitmapDecoder::GetPreview and IWICBitmapDecoder::GetThumbnail operations return the same thumbnail sized image. I am testing this on DNG images created by the DNG converter that contain full sized JPEG previews.
To clarify: The DNG files I have tested with contain both a thumbnail and a full sized preview. The norm is that the GetThumbnail function returns the thumbnail (from the first IFD) and the GetPreview function returns the full sized preview if there is one. The current version of Adobe's DNG codec, however, returns the small thumbnail for both functions.