Google responded to recently reported security concerns with Google Docs, “We quickly investigated, and we believe that these concerns do not pose a significant security risk to our users.” Here’s Google response:

1. Images are maintained because removing them would break image references in users' other Google documents and external blogs.
2. The ability for document collaborators to view revision history is a feature built into Docs. The ability to view past versions of the drawings is limited to authorized persons who have been given explicit access to the document with the embedded drawing. To remove access to revisions, document owener can simply make a new copy of the document (from the File menu) and share that new version. The revision history of both the document and all embedded drawings is removed in copies of documents.
3. The specific case is related to the use of a feature which allows document invitations to be forwarded to more than one person. That feature was provided in response to user requests for "invitation forwarding" and sharing documents with email lists. Invitations sent using this feature contain a special key on the document link. This feature can be disabled at any time to expire previously distributed invitations which contain that special key. To do this, simply disable this feature by unchecking it (in documents and presentations, it's called "invitations may be used by anyone" and in spreadsheets it's "editors can share this item").