Someone should make an ebooks site that uses Dropbox as a backend. Local file control is a really important part of keeping files secure in the cloud. Controled from a single company is really just control with a single point of failure.
I think Kindle works the same way. It's just that they can come and seize any of your books whenever they feel like it. Dropbox will not be much different because if they delete a file on their server and enforce updates on all of your machines, it will lead to the same result.
dropbox not being a single company?
I meant that you'll have a copy on all of your machines.