You bet your sweaty armpits it is.
This year, Labor Day is on Sept. 3, so August's averages apply more than September's. The average maximum temperature in August is 102, with a mean temperature of 88. Las Vegas' record high for September is 113 (set on Sept. 1, 1950), while the record low was 47 (set on Sept. 27, 1948). September's highest low temperature was 67 (9/18/65) and the lowest high temperature was 84 (9/01/02). Note that the highest high and highest low were both in the beginning of the month, while the lowest high was at the very end.
The pools, of course, remain open.
However, if September's stats come into play for Labor Day, it'll be slightly cooler: an average max of 95 and an average minimum of 65 -- meaning hot (though not blistering ) days and mild nights.
Also, keep in mind that Labor Day always falls within Las Vegas' North American monsoon season, which runs from approximately July 15 through Sept. 30, so there's the possibility of a spectacular Southwestern-desert electrical storm, which always cools things down considerably -- for a short while. (See QoD 6/28/07 for the full monsoon story.)
No matter if it's summer hot or fall mild, though, you should plan to bring mostly summer clothes. But pack a sweater if you'll be spending much time inside casinos, which are kept at notoriously arctic temeperatures throughout the heat of the summer.
