Purported Galaxy S8 back and chassis colors tipped
Six of the alleged seven Galaxy S8 chassis colors
If this picture of the Galaxy S8 rear holds water, it will indeed be with an elliptical finger scanner on the back, and the front logo will be moved to the rear cover, too. Samsung apparently thinks that this placement of the finger scanner is where your index finger would naturally land when stretched while you hold the phone with one hand, and it has a point.
In addition, the elliptical shape helps save space, according to the schematic below, as it still encompasses your pointy finger, while a rectangular one would keep a lot of inactive area. Just two weeks left to the official March 29th announcement of the S8, so we'll see if the rumored color schemes and finger scanner placement pan out soon enough.