Worth reading: a smart post by Fred Wilson about data portability as it relates to web apps (gmail, salesforce.com, google calendar, writely, JotSpot, hotmail, typepad, facebook, flickr, youtube, etc.)
Wilson's suggestion: allow users to decide where their data are stored. Most folks will opt for the webapp itself (no cost, no hassle). Users who want additional data independence could pay to locate their data elsewhere (Amazon's S3, a corporate data appliance, etc).
I like Fred's idea of decoupling the app from the storage.
Even without holding the user's data, the web app firm still receives the eyeballs and time on site (and thus the monetization value through advertising, upgrade fees, etc). And the web app would still be quite "sticky" due to difficulty of data migration -- it is usually non-trivial to migrate data from webapp "A" to alternative webapp "B". Consider trying to move gmail archives to hotmail -- that wouldn't be quick or seamless.
But web apps and software-as-a-service would gain more acceptance in the business world companies could park their data whereever they wish.