I "discovered" the MGM Grand parking garage rooftop as a good photo shooting location in December 2000. Like most tourists, I used to walk up & down the strip shooting photos from the sidewalk. After doing this several times over a period of years I began seeking out new angles that would let me capture images that haven't already been captured by every other Las Vegas tourist.
The seventh floor of the MGM Grand parking garage is one of those locations. While it's often inaccessible to cars (it's only open when the space is needed) you can still ride up the elevator or walk up the deactivated escalator from lower floors to the rooftop. Security guards have never hassled me for going up there, so I suspect other photographers are going up there often enough to make it not worth the hassle to enforce some kind of "this floor is closed" rule.
Like most parking garage rooftops, there's a short (three to four feet high) cement wall running around the area, which is about a foot wide on top. If you don't have a tripod with you, simply put your camera down on top of the wall to hold it steady for night shots. This trick doesn't offer you a lot of shooting angles, however. You can rotate left and right but not up or down. So I suggest bringing up a standard sized tripod with you, or at least one of those small tabletop models to use in conjunction with the cement wall.
I only rate this as a good photo shooting location rather than excellent due to the limited views that are available.
First of all, you could shoot the MGM main room tower itself; but that's the one thing security will hassle you about. They don't care if you shoot photos, but they will tell you not to shoot towards the west; which is a guest room building. This happened to me while I was shooting pictures of the Luxor... even though I wasn't shooting the guest room tower, the angle was close enough to the MGM tower that security came up and told me not to shoot in that direction. I offered the guy $20 but he wouldn't go for it.
Their concern is that guests think you're spying on them. Although you'd have to have some very good equipment to capture images of guests in their rooms from the parking garage roof, the guests don't know that. So it's a valid rule to enforce.
You can get some good shots of the Luxor from up there, so I suggest you get those FIRST, before security has a chance to hassle you with accusations of spying on guests. They'll either come up and tell you to shoot in other directions, or not bother you at all (I've shot the Luxor from up there several times and was only reprimanded for it that one time; so it may not be an MGM policy but rather the opinion of individual security guards as to whether or not photographers should be allowed to point their cameras towards the west.) Technically, you could be ejected for trespassing if you're not an MGM guest; so be cooperative.
In addition to the Luxor, you can get decent shots of Hooters (which is for sale as I'm writing this, so it may be something else when you read this) and the Stratosphere tower, which is about four miles away.
Walking over to the north side of the rooftop, you can get some shots of the back of a few properties (the Planet Hollywood (formerly Aladdin) - Bally's - Paris area); a partially obstructed shot of Rio Suites; and the monorail line.
The east side of the rooftop offers a view of the Hard Rock, but even with 10x (approx 300 mm) zoom, I couldn't get a good shot of it. So you'll need a lot of zoom if you want to get that picture.
The south side offers the Hooters view and not much else along Tropicana Avenue.
Below I've posted some of the photos I've taken from the MGM Grand parking garage rooftop. The view has changed slightly over time; for example, in the first photo you can see a green contraption which was part of some kind of thrill ride in MGMs amusement park, which no longer exists.
