As far as I know there is now way to tackle that problem through Google Apps itself. There are a couple ways (like offline storage) that you can mitigate the problem by hoping someone else besides the disgruntled owner has a backup on their local machine or something like that, but that's unreliable.

Your best bet is to use the API to periodically search for newly created or modified documents and download them for backup. You can query for things like a minimum updated date, for example.

Documentation: [code.google.com]

I've considered doing something like this at our company. My idea was to run a nightly cron job that uses the API to essentially change ownership of newly created documents and then shared them with the original creator so that their documents become company documents which they can edit. The idea never made it anywhere near the top of my TODO list though.