I've seen this and I have to admit I'm pretty impressed:
http://gmailblog.blogspot.com/2010/04/drag-and-drop-attachments-onto-messages.html
I'm almost certain that this is an HTML 5 feature (it's supported on Chrome and Firefox 3.5), and was wondering if somebody knew how is the browser feature called.
Bonus points for a tutorial/article on how to do something similar!