What you're referring to is the Plexiglass cutout at the end of the Ghost Bar patio, located on the 55th floor (actually the 35th floor, since the Palms skips several numbers for "superstitious" reasons -- see QoD 07/04/05), about 450 feet above the ground. It's just a small area, just large enough to stand on and see down, but ever since the property opened back in 2001, it's been a magnet for partiers who want to stand, smooch, or jump up and down on it.
As many of said partygoers wear spiked heels, this necessitates frequent changing of the Plexiglass, which tends to get scuffed up and scratched on a regular basis. If you're lucky enough to get there when the view's nice and clear, definitely do not stand on it if you suffer from vertigo -- it's a long way down, especially when you're looking directly through what you're standing on. (You could do it as a trial run before visiting the Grand Canyon Skywalk, to see if you're up to a 4,000-foot drop.)
You can see it in the picture below -- it's the white-colored rectangle toward the back of the patio (click on the link for a larger version).
It reminds the current writer of a bar she used to frequent in native England, which was built on the site of a former crypt (hence its name, The Crypt). This was back in the mid-'80s when the whole Goth thing was big, so it was a particular favorite of ours not just because of the name, but because it had a similar Plexiglass cutout in the floor, only this one was longer and wider so that you had a good view of the skeleton lying underneath you!
