I recently had to move a bunch of files from a free Google Docs account to a Google Apps Premier account. Exporting documents one by one wasn't an option. There were too many files for that. I needed some sort of batch solution.
After a lot of trial and error -- and fruitless searching -- here's the process I came up. This works the same between free-to-paid and free-to-free Google Docs accounts.
Note: This is a workaround, and a somewhat clunky one at that. It does not address Google Docs' major problems. If you're trying to apply this in a corporate or enterprise environment, be sure to consult the caveats at the end of the piece. This may not work for you.
What you'll need
Two Google Docs accounts -- I suggest opening your first account in Firefox and the second account in a different browser (Safari, IE, Chrome, etc.). As you go through this process, you should also regularly check the account name in the upper-right corner of the browser window. Juggling accounts can be confusing.
Steps for moving files between Google Docs accounts
1. Create a transfer folder on your first google docs account by going to Create new > Folder. (I called mine "Stuff to Transfer.")
2. Click on All items. Click the checkmarks next to the files you want to move.
3. Click on the Folders button. Select the transfer folder you created moments ago. Click Apply changes. Your files will now be associated with your transfer folder.
4. Click on My folders (left side). All of your folders will appear in the right window.
5. Click the checkbox next to your transfer folder. Go to Share > Invite people.
6. In the invite box, type the email address associated with your second Google Docs account.
7. Click the Add without sending invitation link. You'll see a dialog box with a message about skipping invitations. Click OK.
8. The Share with others menu will reappear. Under the People with access tab, you should now see the email address associated with your second Google Docs account. Click the Save & Close button.
9. Go to a separate browser and log in to your second Google Docs account. Click on My folders.
10. Go to Create New > Folder. This is where you'll be transferring your files. Name it whatever you like (I called mine "Transferred Files"). Click Save.
11. On the left-hand side, expand the Folders shared with me option. The folder you shared from your first Google Docs account should appear. If it doesn't, refresh the page. You may also need to close My folders to reveal the shared folders option below it.
12. Under Folders shared with me, click the shared folder from your first Google Docs account. The shared files will appear in the right window.
13. Click checkmarks next to any files you want to transfer to your second Google Docs account.
14. Click the Move to button and select the folder on your second Google Docs account where you'd like to transfer the files (this is the folder you created a moment ago). Click Move to folder.
Your files will now move from the shared folder to the new folder you created on your second Google Docs account.
The big, ugly caveats
The original versions of your transferred files are still in your first Google Docs account. All you've really done is move files out of the transfer folder you created at the beginning of this process. The files haven't been deleted.
Speaking of deletion ...
A big important note on deleting and "unsharing" files: As msilver notes in the comments, do not delete files from the original Google Docs account. Removing sharing at the document level is also problematic. Why? Because deletion/unsharing removes access to the files from the second Google Docs account. This is a huge problem because it means you have to keep your original Google Docs account around. Bottom line: share all you like, but don't delete or unshare the original files.
Another unfortunate thing: Google Docs allows for transfer of file ownership, but the new owner has to be within the same domain. That's fine if both of your Google Docs accounts are associated with Gmail addresses. I swapped file ownership between two free Google Docs accounts without incident. But this domain limitation presents a huge problem on the free-to-paid side. For example, I cannot transfer ownership of a document created in my free Google Docs account over to my paid Premier account.
The only solution -- and it's not great -- is to keep files in your original Google Docs account. I'm choosing to call that my archive (how's that for a silver lining?). I can access and edit all my material from the Premier account, so it's not a huge deal for me. But companies that want to transfer content from employees' personal Google Docs accounts are out of luck. Perhaps the launch of the Apps Marketplace will catalyze some much needed and long overdue enterprise development.