First you'll need to view the headers of the mail. To do that, open the message, then click "More options", and finally click "Show original". Look for the lines with "Received: " in front of them, and then start reading them from the bottom up. The IP of the sender should be in there somewhere. There are more detailed instructions on the web if you search...
http://answers.google.com/answers/threadview?id=139415
http://www.stopspam.org/email/headers.html
Here's an example of a header from a message sent by the forums:
code:
Received: from mountain-dew.avidmotion.net ([216.127.82.64])
by mx.gmail.com with ESMTP id 73si1352564rnb;
Wed, 25 Aug 2004 21:37:45 -0700 (PDT)