http://gmail.google.com/gmail/help/more.html
Data retention
Some news stories have suggested that Google intends to keep copies of
users' email messages even after they've deleted them, or closed their
accounts. This is simply not true. Google keeps multiple backup copies
of users' emails so that we can recover messages and restore accounts in
case of errors or system failure. Even if a message has been deleted or
an account is no longer active, messages may remain on our backup
systems for some period of time. This is standard practice in the email
industry, which Gmail and other major webmail services follow in order
to provide a reliable service for users. We will make reasonable efforts
to remove deleted information from our systems as quickly as is
practical