Right side of the Nexus S 4G
The power button is found on the right side of the device.
I just got the Sprint version of the Nexus S this weekend. So far, so good. I'll post a review this week on http://trial-technology.blogspot.com/
I've used the Nexus S for over a week now, and have shared my review: http://trial-technology.blogspot.com/2011/05/samsung-nexus-s-blackberry-replacement.html