This is fantastic. This is exactly how I foresee the future documents and editing. Dropbox, or something like that, provides the service of saving the files. Then web apps are granted access to them and can edit them. Local storage becomes a cache, and Dropbox is where the files are "really" stored.
The advantage of this, of course, is that all your stuff becomes device-independent. Log in from any device that has web access, and you'll have access to all your apps and all your files.
This is much better than having each web app manage files on their own. For instance, you might have documents saved in Google Docs, notes saved in Evernote and Catch.com, and emails saved somewhere else, etc. It sure would be nice to have a single service provide the file storage so that you always have all your stuff and don't have to worry about some web app going obsolete and losing your data along with it.
Now what I would really like for backwards-compatibility is a web-based Linux environment to access my files. Someone recently created a web app using the HTML 5 canvas element (I think) to run an X GUI. (Anyone remember what this was called and where it is located?) Attach a Linux system and that GUI technology to your Dropbox, and then you have an entire computer that can be accessed from any device. Either the browser itself would load all the files and run them by emulation, or a server could run them natively and just print the results to your browser. Either way, once that becomes possible, I would be able to convert to an all-web environment.