I'm curious -- if an electric guitar amplifier has a two-conductor power supply cord (with hot and neutral conductors only) and a "ground lift" switch, what does the switch do?

Can you provide an example make and model that has a schematic available on the web? For example:

http://www.thevintagesound.com/ffg/

I don't know about ground switch, but some of them had a "hum" switch or something similar... flipped the ac lines. Sometimes the "neutral" was connected to chassis... which with no keying on the ac plug meant the chassis might just as easy be hot. Nobody has done that for a long time now. The idea was old and outdated 50 years ago. (probably before)

One side of the power line coming in to the amp is tied to the chassis through a capacitor, the ground switch changes which one. This is what is known as the 'death cap' in amp-dude circles.

When it dries out and fails (not if, when), the chassis will be hot when the switch is in one position.

It's labeled 'ground switch' on most of the Fender schematics at that link, between the mains input and the power transformer. Most amps were made this way.