Stopped by my local store earlier around 5PM, and the Samsung area was deserted, maybe the guy was gone to dinner. All signage and displays were still S7 related, except for a small TV on the counter that was playing a short looping S8 video.
I'm in the Minneapolis market. The 3 Best Buy locations I checked with were all displaying them today.
I spent 30 minutes playing with both versions tonight at my local Best Buy. The build quality is TOP NOTCH - I was expecting it to be good based on everything I heard...but I was still thoroughly impressed - something that doesn't usually happen to me.
I can't get over how small they feel in your hand. I was planning to buy the S8 but after seeing them in person, the S8+ didn't (to my admittedly large hands) seem big at all. I'm currently lugging around an iPhone 7 Plus and before that I've carried every Note ever made. By far the S8 and S8+ are the most comfortable phones I've held in my hands. They are incredibly well built and stealthy in their size.