DropBox should work already via DropBox's own software.
I don't know much about SkyDrive or Amazon S3 beyond the basics.
We have a plugin API which would let anyone interested enough in those things write the support themselves, and we'd be happy to provide help to anyone who wanted to do so.
It's not something we are looking at ourselves, at least at this time, since it would require a lot of time & effort for each individual service, and there is no clear winner right now. We'd either spend a huge amount of time trying to support them all, or only please a tiny handful of people, neither of which seem worth it to us at this time.
(BTW, for unrelated questions, please start a new thread to help people looking for similar answers. Or continue an existing one if there is one (e.g. here or here.)