The Hokuyo URG-04LX laser range finder is pretty popular, and today I noticed someone was wondering where the optical center is. Well it turns out that there are actually two optical planes, one for transmit and one for receive. Since the alternative is probably dealing with non-euclidean geometry, for my robots I generally use the average between the two as the optical center and then this assumption gets wrapped up in the error term. I found this information somewhere on Hokuyo's website in their password protected customer area.
As an added bonus here is a 3D cad model (right click save as) of the Hokuyo URG-04LX Laser ranger in STEP Format. The CAD model and other measures is not guaranteed to be accurate.