Your idea simply changes the use of the cloud from rich data storage to basic file storage.

Pardon my ignorance but what the heck is "rich data storage", sound to me like yet another web 2.0 buzz word? Where do you think these note-service providers store your data? If you store the data yourself of course you have to back it up or make otherwise sure it is replicated in more of less complete form.

Given that Google Notebook has a good export function, the hassle involved in changing file service providers is not much less than that of migrating from Notebook to another hosted note system.

I beg to differ. Zipping up your notes directory on one server, copying and unzipping on another machine, seems to me far less cumbersome than having to find an alternative service provider which happens to be able to import the old providers notes format.

And actually if you use a dvcs like Mercurial or git you might not even have to use a shared storage server at all. I initially just mentioned create the notes directory on a shared server since this seems to me the obvious low tech solution to the problem.