Asmith, good point.

While Larry is correct about it being state to state, my google search reveals most states require it.
Excerpt from CA Vehicle Code:
Reflectors on Rear24607. Every vehicle subject to registration under this code shall at all times be equipped with red reflectors mounted on the rear as follows:
(a) Every vehicle shall be equipped with at least one reflector so maintained as to be plainly visible at night from all distances within 350 to 100 feet from the vehicle when directly in front of the lawful upper headlamp beams.(b) Every vehicle,
other than a motorcycle or a low-speed vehicle , manufactured and first registered on or after January 1, 1965, shall be equipped with at least two reflectors meeting the visibility requirements of subdivision (a), except that trailers and semitrailers manufactured after July 23, 1973, that are less than 30 inches wide, may be equipped with one reflector which shall be mounted at or near the vertical centerline of the trailer. If the vehicle is equipped with two reflectors, they shall be mounted as specified in subdivision (d).
...
(f) Area reflectorizing material may be used in lieu of the reflectors required or permitted in subdivisions (a), (b), (c), (d), and (e), provided each installation is of sufficient size to meet the photometric requirement for those reflectors.
Need to check if tail lights themselves (on all the time) comply or are at least reflective....