Well, according to Yahoo!'s research folks, here is the origin of the term 'spam' for junk E-mail:

http://ask.yahoo.com/ask/20030228.html

NODE agrees with the Yahoo story AnnaS, albeit with a small alteration in the detail.

ORIGIN 1930s: apparently from sp(iced h)am. The Internet sense apparently derives from a sketch by the British ‘Monty Python’ comedy group, set in a café in which every item on the menu includes spam