My air conditioning quit working last night. The service man showed me that the current available to the unit was only 100 volts. He also said that the neutral wire was hot. He could not fix this because he's NOT an electrician. I'm not either but while I wait for the electrician to show up, I'd like to understand how this could happen as well as what to expect for a remedy.

The Neutral draws the same as the Hot, They should show same heat damage unless something like neutral screw loose or neutral loose to recpticle.
Replace recpticle and Ac plug. Loose connections generate heat. If you replace one, and the other still corroded/burnt you will be in a circle. Make sure screws are tight.

I am sure you meant a single white wire to recpticle. If you meant at panel. Could be multi-circuit and 2 wires sharing a neutral (must be on opposite phase)If one circuit draws
20 amps and the other draws 20 amps, the neutral will draw 0.
However, If both breakers are on same phase(Same Big wire, hot)Every other space is opposite phase.

Oh yea, almost forgot, measure voltage at ac recpticle(100 volts), measure voltage at breaker.If qualified, remove breaker and look at bus and breaker connecting . Is either burnt?
Last, make sure Neutral is tight
Personally, I would also verify tightness of all breaker and neutral connections

When I say the neutral wire is HOT, I don't mean temperature.. . I mean voltage

This is a central air conditioning wired directly from the breaker. Only going through a simple switch in the closet next to the unit. I assume the only appliance on this circuit is the A/C. With the black and white wires disconnected from the A/C unit.. . I've only got 100 volts. I have not yet checked the voltage at the breaker. I'm curious what could be causing the loss of voltage

It may just be a bad breaker. There shouldn't be power coming through the neutral, because there isn't a neutral wire at the outside unit. You have 2 lines both to ground should have 120 volts test them together you should have 240 volts. Again I'd say you lost the breaker.

Turned out the problem was a loose neutral wire in the buss. The screw itself was tight but that turned out to be a false positive because it was not tight against the neutral wires it was supposed to be holding in place. The wires were quite loose. Moving them to another hole and tightening the screw fixed the problem. Thanks guys for your advise. I have air conditioning again and it's soooo cool. ; )