Does anybody know how long Google retains deleted messages and/or attachments on its back up servers?

That's easy.. 30 days.

https://mail.google.com/support/bin/answer.py?answer=13288

I don't think there's an official answer for your question.

"You may organize or delete your messages through your Gmail account or terminate your account through the Google Account section of Gmail settings. Such deletions or terminations will take immediate effect in your account view. Residual copies of deleted messages and accounts may take up to 60 days to be deleted from our active servers and may remain in our offline backup systems."

http://www.google.com/mail/help/privacy.html