I've been working with the Dropbox API (among MANY others...) for my early stage startup Snapier (http://snapier.com/), and I must say it is a pleasure to work with. Very sensible and logical. I can't say that about all API's...
Another example of how Dropbox is gonna keep its momentum. With it, it should be rather straightforward to bake in some cool iCloud type of preview/revision stuff into native apps.