I just bought the 4S yesterday (also my first iPhone) and I have a similar hot spot in the exact same area and of roughly the same dimensions. It's not as pronounced as yours (i.e., mine doesn't appear to show up on video or stills) but I can detect it once in a while.
Oddly, it seems to come and go and may be related to the heating up of the phone or ambient temp.
Either way, like you, I'm not inclined to return it. I've been to that dance before with countless MBPs and iPads and inevitably wished I could have just gone back to the one I first had.
With no battery issues, dead pixels, backlight bleed, chips, scratches, camera noise, etc., I'm happy with this one.
Could you give us the first 3 letters or numbers in your serial number