From the Internet's perspective, your computer was never on the Internet at all. If someone traces the connections back, the packets will all look like they came from the hijacked connection. This is especially useful when dealing with monitored conversations (see Echelon, Carnivore, the ultra-secret level-3 DeadMan monitoring stations that pepper the globe like icepick wounds in Trotsky's skull).
Note that I'm talking about taking over lower-level connections, *not* impersonating users to make it look like they sent an email or a Usenet post. That sort of stuff is relatively trivial. I'm talking about physical and data-link OSI-level takeover. I was going to use this very technique to send this post, but it takes finding a physical close computer that is running Win9x or “Win2K”, and the scanning program to find such a computer has not been ported to Windows 2000 (which is the program I am using to compose this article).)
Alien Internet Conspriracy [1]
Uh, yea. Right.