In characteristic Las Vegas Sands fashion, we got blown off by their PR people when we asked about this. However, Stiffs & Georges blogger David McKee believes this to be an urban legend. He participated in a 1999 pre-opening tour of the property, during which Sheldon Adelson praised the "excruciating detail" with which the property had been executed.
Although he waxed eloquent on such niceties as carved toenails on the Venetian’s statues of cherubs, Adelson made no mention of a Rat Pack plaque. (His demolition of the Sands was still a sore point with Las Vegans.) Nor do guidebooks or Vegas-nostalgia websites suggest that such a marker exists. One place where you will find Rat Pack plaques is the Golden Steer Steakhouse, 308 W. Sahara Ave., where their favorite booths are denoted (see QoD 06/27/2010). Since it dates from 1958, the Golden Steer will give you a far better "throwback" experience than the somewhat Disneyland-like Venetian.