please, every handset maker uses different techniques. each of those handsets are built differently. i would say the antenna placement on the latest iphone makes it faulty, however what does that add to this discussion?
if you continually go in and out of dim/bright locations the brightness increases/decreases, which uses more energy over the screen being one set brightness level. in addition, the photo diode that "looks" at light levels has to be energized constantly. one thing ive noticed with the 3D is the brightness changes are smoother whereas the evo 4G has less steps in between light level change so the changes would be more noticeable albeit not DRAMATIC...
to move this discussion forward, if one feels its an "issue" or "defect" is encouraged to e-mail HTC and go into a corp store and have them check it. have them test it against another handset/demo headset in the store. just posting here asking if others see it is one way to gauge what people think, however take it a step further and contact the manufacturer and Sprint.