This seems like a really useful utility!
My first experiment was to create a 'DropBox' folder on my desktop. I then created a bunch of rules that would move files to my various document folders based on their filename and/or extension. E.g. any files with a .xls extension get moved to my Excel folder. This means I can save or drag everything to my DropBox and have it automatically filed. Over time I will expect to develop a file naming convention to deal with more complex filing rules, but for now it's working pretty well.