Galaxy S8/Note 8 finger scanner underneath the camera lens for the Galaxy S9 indeed, if a leaked screenshot, allegedly detailing how to measure your heart rhythm in the S Health app is to be believed. As you can see in the grab, Samsung's heart rate sensor that is used in conjunction with the S Health app, is positioned next to the camera on one yet-to-be-revealed handset, while the notorious elliptical fingerprint reader of the Galaxy S8 and Note 8 is situated way below, and under the camera.
This is certainly a better solution than having to reach across the camera lens to use the scanner on Samsung's 2017 flagships, risking smudges, at least in the beginning while you are getting used to it. The fingerprint reader on this screenshot here seems to be right where it belongs, in the middle of the back, though it's still not big and round, so there could be a learning curve until one hits the exact index finger stretch amount needed to hit it without fingerprinting the camera lens.
source: Keyforweb.it (translated)