IPhone :: What Is Pwr On / Off Screen Similar To Incoming Call Screen

I purchased a new 32gb iPhone 4 from my local Apple Store. After everything was all setup, I walked around the store trying to replicate the "death grip" which I was able to do, mostly anyway. A few seconds go by and a screen pops up, very similar to the incoming call screen.

It was plank at the top where the number usually would go, and below that it said "Pwr on/off". At the bottom was the usual Accept / Decline buttons. The Apple genius immediately said "Oh, hold on a sec" and pressed the "Accept" button, and the screen disappeared. When I asked him about it, he was very very coy about it and said "Oh, it was a diagnostic screen, don't worry about it. It's gone and you won't see it again." I kept asking what the screen was and he kept deflecting the question.

Anyways, much of the screen description was coming from memory, so I'm not 100% sure it was exactly, but it was damn close. I wish I had time to take a screenshot. Anyways, has anyone else seen this screen before? What is it?