As far as I know there are no "national" requirements. I could be wrong, but I think that each state is allowed to set its own regulations on the details of inspections and requirements (up to a point). But I know that, as long as you satisfy your own state's requirements, you can drive the car in any state you want (of course).
Here is some info that I was able to scare up quickly:
Pennsylvania Lift Laws | Vehicle Modification Rules
-- Says that in Pennsylvania, the minimum height requirement is 24 inches.
Height Regulations for Headlights | eHow.com
-- Says that the following states have a minimum height of 24 inches: Alaska, Arkansas, Washington DC, Indiana, Kansas, Louisiana, Maine, New Mexico, New York, Oklahoma, Oregon and Texas.
-- Interestingly, I think this is a copy of our actual current vehicle code in Pennsylvania, and it doesn't say anything about height requirements that I can find.
-- Oh, wait, here is a list of tables that apparently go along with PA's vehicle requirement code. There's some very interesting stuff in here....stuff I kinda don't want to know about.
But it again states that 24" is the minimum. (I'm starting to not like the answers that I'm finding).
Anyway, I think for any given state, if you Google "minimum headlight height" you can find a specific answer. (You're here in PA, too, aren't you Dale?)
But here's the thing...don't go out and measure your headlights. You probably won't like the answer. My experience has been that our headlights are at best just barely able to meet the requirement and the inspectors realize that they are "unaltered" on a car that is about 30-40 years old and has existed in every state. In other words, unless you were silly enough to lower them even further, I think they'll pass inspection. Don't measure yours too closely. Look down at them at an angle and let parallax give you the answer you want.
The Sterling and the first generation Cimbria have headlights that are inset into the front -- the ones in the first gen Cimbria are behind a door that folds DOWNWARDS -- and therefore these to cars represent the lowest of a low. The Cimbria SS, the ultra-rare Sterling "GT", and both versions of the Sebring had pop-UP headlights.
The rumor is that one of the reasons that the Sebring had pop-up headlights was to make certain that it met the minimum headlight requirements.