Cool, I've not heard about Hackpad before. We are closely watching developments from Google, Microsoft and Apple in the collaborative/cloud writing space. Let's see what Dropbox makes out of Hackpad. If they make it into something mainstream with a good API we would love to integrate with it.
More generally, it will be very interesting in what direction Dropbox will move. Online storage will be a commodity very soon with Google and Microsoft already offering unlimited space with their office products. So it seems Dropbox will have to come up with something to compete in the long term. They're great product people so they might be able to offer a real alternative to Google Docs, MS Word and Apple pages.