One of the features that I found very useful in Adobe Bridge was the ability to group images into a stack. For example: 1001.dng, 1001.tiff,1001.psd and 1001.jpg could all be grouped together. Simply select the files and press ctrl+G to group. This removes clutter and makes for easier viewing of the photo files since I need only look at the top photo in the group to recognize it. A modifier key or mouse click could be used to expand the group, allowing for further editing of a specific file within the group.
Unless I am not seeing it, I think it would be a nice feature to add to PM.
Mike