In this plugin, The droppable file count should be limited to one, but it may be hard to do since I think there is minor bug (or misunderstanding) in the plupload js-plugin that this is based on. It seems to ignore the "multi_selection: false," setting.
Right now, in this plugin, if I selected 4 images and dropped them onto the droparea, all of them get attached to the post and the last one becomes the thumbnail. I guess that's OK as long as the user understands that.
That multi-file drop ability is the default for the WP version 3.5 media uploader (which is vastly improved IMHO).
By the way, in order to implement this D&D functionality on a user-facing submit form in a page template (like a contact form), I'm digging in to the plupload API (and ajax and jCrop which is stretching my comprehension of js).
Thanks Tobias for the inspiration, and I will share my code with you when I get it all working.