Though we won't get an official side-by-side comparisons of the camera in the iPhone 4S and the new iPhone 5 until we get our hands on it, this is about as close as it gets: Scott Everett, from Dpreview, took nearly the same picture of Big Sur that Apple has been using in its iPhone 5 sample gallery with his own iPhone 4S. It's the first head to head look of the camera in the iPhone 5 and iPhone 4S.
It's a remarkable coincidence that's astutely analyzed by Dpreview and they've discovered that the iPhone 5 (though listed as having a similar spec'd camera) actually has a different sensor than the 4S. Dpreview compared the EXIF data and saw that the iPhone 5 was able to drop to ISO 50, which means "for images taken in such bright light, this strongly suggests the newer model has a lower minimum sensitivity." Also, Dpreview suggests that the iPhone 5 has a slightly larger sensor than the iPhone 4S when digging deeper into the EXIF data.
From our perspective, there seems to be a little more detail in the iPhone 5 but that could be a lot of different factors in play (time of day, etc.). I definitely prefer the look of the iPhone 5 camera more but on first glance, it doesn't seem as significant a jump as the the iPhone 4 camera to the 4S camera was (and definitely not 3GS to 4). Of course, this isn't a real hands-on comparison and it's not taken at night where we can see if the iPhone 5 has improved low light performance, but it does give you the idea of what the 4S and 5 are capable of. Check out the real photos of the two iPhones at Dpreview.
Update: Gizmodo reader Darshan Meda just sent me the picture he took of Big Sur with an iPhone 4 and I've added it in for reference. Again, there will be obvious differences (given the different times each picture was taken) but you get the general gist