Ok, well after playing with my laser level for a bit, it looks like this.
No keystoning: Angle the projector so my laser level line goes through the center of the screen, through the center of the lens, and through the center of the rear of the projector.
Keystoning: Angle the projector so the laser line goes through the center of the screen, through the center of the lens, and then straight down the projector (thus ending up a little off center on the projector because the lens is off center).
So, it appears that to get a clean image, I need to have my projector angled ever so slightly to the screen, though the angle causes the laser line to go through the dead center of the rear of the projector. I suppose if the optics are in the center of the rear of the projector, that explains why the lens doesn't come straight out of the projector, and why I have to have it angled. Maybe this is a stupid question, but why would they put the lens off-center the optics on-center, and then angle the lens?
Again, I'm a newbie here, so tell me if I'm making some fundamental mistake here