Oh... a DC/DC story, well... I have to have some comment on that.
Hausman is probably right - DC/DC converters are usually done in that manner - there are a couple of ways to ensure protection on these. Good DC/DC converters are even protected from the short circuit (high current, zero voltage - standard AC/DC adapters usually end up with a fried transformer after applying short circuit on the DC side). In contrast to the standard AC/DC adapters (linear, open-loop circuit configuration), DC/DC converters use negative feedback and a totally non-linear circuit (switching) configuration, so they are actually very power efficient (70-95%).
Usually, DC/DC converters have a raw converter (power switches (diode, MOSFET, IGBT, thyristor...), transformer, inductor, capacitor) and a PWM (Pulse Width Modulation) driving circuit which, on good DC/DCs, is very smart (has sensing for the output voltage and protection in case of a short circuit), but since these are extremely non-linear circuits, and we're talking about extreme amount of power during transition periods (peak current at 15 amps, if normal current is at 4 amps), Zener diodes are not used as a protection ('cause you'll need a very big Zener diode if you don't want it to get fried first time you plug it in the AC). Besides, Zener diodes are mainly used as a component in linear voltage stabilizers, and have been abandoned for any serious use because of the noise they produce (white Gauss noise), so there's no place for Zeners in a Thinkpad power supply
So... if we exclude the possibility that IBM engineers have developed a dedicated modulation type so they could insert the adapter info into DC lines, and totally forgetting that they could only add one more wire to do so... yes, the output voltage goes down as soon as inductor hits i_max, in order not to fry anything.
Also HP uses the system where they have like standard 18.4V for 65W adapter and 19V for a 90W (0.6 volts is enough to see which adapter you're using) on the same type of plug - even for the same type of laptop (but one model has integrated video, another has dedicated) - HP nc6400 for example.