Several Gmail users have lost e-mails according to GigaOM.
A statement from Google regarding the matter:
"Regretfully, a small number of our users — around 60 — lost some or all of their email received prior to December 18th. Once we found out about this issue, we worked day and night to confirm that only a few accounts were affected and to do whatever we could to restore as much of the users' accounts as we could. We've also reached out to the people who were affected to apologize and to work with them to restore the email from any personal backup they might have. We know how important Gmail is to our users – we use it ourselves for our corporate email. We have extensive safeguards in place to protect email stored with Gmail and we are confident that this is a small and isolated incident."
If you store a lot of important stuff in your Gmail account, my advice is to download all e-mails using a POP client. (I highly recommend Thunderbird.)