There is a new feature in Google Docs where it is possible to download all of your documents as a zip file.
Right click on any document in Google Docs, select "Download" from the pop up menu.
Change tabs from "Selected Items" to "All Items" and configure how you would like your different documents to be downloaded.
Click on the "Download" button, it might take a few minutes to prepare the ZIP file if you have a large volume of documents.
(This seems to work for both traditional Google accounts and Google Apps Premium accounts, the only ones I've got available to test)