Some semis (18-wheel lorries) have a light on in the space between the cab and the trailer. Most of the semis you'll see will not have a light there, but some will. I assume there is some meaning behind this, but I could be wrong.
Driving back from CT on 81 South at night (on Aug 15, 2004), I counted the semis heading north (odds are very low I'd count the same truck twice). Out of 238 semis, 17 had a red cab-trailer light, and 1 had a green cab-trailer light, the remainder were dark (no light).
I've heard that the lights indicate the sexual preference of the trucker, but the only mention of that I've found is some newbie, and truckchaser's howto (warning: sexually explicit if they ever fix their graphics links) makes no mention of it; so it seems improbable that it is a sexual-preference indicator.
What the heck do those lights mean?