Ilia Alshanetsky has created a simple script (that uses curl) to make a backup of your Google Docs to keep you and your data a bit safer (since Google provides no backup capabilities).
As part of the backup strategy we also wanted to capture incremental versions of the documents (on a daily basis) in the event we needed to go back to the prior versions. To this affect I whipped up a small (120 lines) PHP script that will retrieve all your Google documents and save them to a local directory, in the event the document was created/updated in the last 24 hours, thus ensuring snapshot support.
His script exports each of the documents contained in the account and puts them on the local file system prefixed with a year/month/day value to keep things unique. You'll need curl and SimpleXML enabled to be able to use the script, but it's a pretty simple thing to drop in and run on most PHP5 installations.