I use Google Docs a lot. I'm rather alarmed by the fact that anyone can pop a Google Doc into my document list. Here's an example where I send a document into an email address I don't have in my Google Contacts list - and email address which, I assume, Google has no idea I can control.
The first step is easy enough. Create the document, share it and then opt for the "skip invitation part".
Then look what happens.
The document turns up in the documents list for the email address I've targeted.
The document itself could contain any message I wanted. This one has a hyperlink but it could also have an image.
Clearly, this isn't a problem right now. Google's fighting to establish Google Docs as a possible replacement/supplement to Office and that's probably why it is so easy to fire around Google Docs, Spreadsheets and Presentations. It seems more than likely that Google will have to crack down on the invitation process at someone point.
I could draw connections between this and Yahoo Groups. In the old days Yahoo Groups moderators would add just any old email address to their mailing list and then use the list to mass mail spam.