Although it is not yet mentioned on the ‘New features’ page, Google have added a ‘report phishing’ link to gmail. It’s under the more options link when viewing an email.
I forgot to mention a while ago that they started marking emails with from addresses at gmail.com, but which were not sent through gmail with the following.
Warning: This message may not be from whom it claims to be. Beware of following any links in it or of providing the sender with any personal information.
And then they don’t make all the links in the email clickable anyway!
Unfortunately, this one is applied too rigorously for me, or at least the rules are too simple minded. For exampe, anything on a mailing list from someone with a gmail account gets flagged thus.