The IP address of the email's sender can be found in the email header data, which is usually not shown by most email clients. To see the header in gmail you can click on the option "show original"; this will show the email's header. Inside the header, look for the following line:
Received: from [70.250.115.165] by web81302.mail.mud.yahoo.com via HTTP; Wed, 23 Sep 2009 20:36:40 PDT. Here, the sender's IP address is 70.250.115.165 (which happens to belong to SBC/ATT).
In order to figure out who is assigned that IP, simply go to one of the myriad IP finder sites, such as http://www.ip-adress.com.
Hope this helps.
Cheers,
Yojinbo