Just played this for the first time tonight. I dunno, maybe all the negative comments prepared me to expect the worst, but I actually really like the lighting. I played thru the whole first level (the jungle area) and the lighting looks very natural. It didn't stand out as wrong or overdone as it so often does in other games. I think they did a very nice job of it. Screen shots don't really tell the tale, you need to see the game in motion on a proper tv.
I say 'proper tv' because I suspect peoples tv's will make a huge difference as to how they perceive the graphics. I admit, I hate lcd tv's, they always look very harsh and unnatural to me. We already had a 50" Panasonic plasma which we love, and had tried a 'high quality' lcd afterwards. I hated it, returned it and got another plasma. I think plasma just gives a much more natural picture and preserves detail better especially for video games where you can have some dark areas on screen that need to keep their detail, but often end up all black on lcd's.
For those that hate the lighting in Halo 3, I'd be interested in knowing what type of tv they have.