Yeah that's more than likely just the power button since most other Samsung smartphones have their power button on the top right (such as my Epic4G) as opposed to the top like others. Just weird that the T-Mobile variant doesn't appear to have one. But maybe just an image editing thing.
@RyTyFern - Yeah it appears to be the proximity and light sensor to me along with the FFC. Let's hope it does come with an LED notification light, and one that works a lot better than that on the Epic4G (mine's ****).