After several attempts, using different computers, terminal software and cables, I have still not been able to make any computer connect to the Plug over the serial interface.
Installing CDM made the computer (Win XP) stop complaining about not recognizing the Plug as a valid USB device, so I must be doing something right.
The Plug is not dead, since I can ssh into it over the LAN, but in order to do what I want (i.e. boot it of an SD card), I have to use the serial interface instead, since someone decided to design the Plug in such a way that changing the boot order has to be done on the fly during boot-up, before ssh:ing into the Plug is even possible.
(In IBM lingo, the plug is "Broken As Designed", see http://catb.org/jargon/html/B/BAD.html
Does anyone have any suggestions as to how one can make the PC recognize the Plug, or is there some way to reprogram the boot order over ssh?
If not, I have simply bought a very expensive plastic brick.