The iPhone generally tends to expose slightly brighter than the camera, but it also seems to compensate in the midtones, expanding the amount of detail you see there. The place it loses is in the highlights--it seems to overexpose them a lot more. Also, if you look at leaves in the upper corners of the iPhone's image you can see the purple fringing.
This is one case where I think the point-and-shoot has an advantage over the iPhone. While the iPhone has a little better white balance, the iPhone's photos are a lot noisier--and that's at ISO 800, compared to the 100 HS's shot at ISO 1600. That said, neither camera is great. The 100 HS is slightly less sharp and displays some color noise.
(iPhone: 1/15 sec, f2.4, ISO 800, pattern metering, AWB. 100 HS: 1/20 sec, f2.8, ISO 1600, evaluative metering, AWB.)
Lack of optical zoom is the iPhone's greatest weakness as a camera. Even the 100 HS' optical zoom doesn't reach far enough, and so I kicked up the digital zoom a little for comparison.
(Sorry, forgot to label these. Left: 100 HS with 2x digital zoom; middle: iPhone 4S with maximum digital zoom; right: 310 HS with optical zoom.)
Neither of the cameras handle backlit scenes very well. (Technically, this isn't backlit but it poses a similar exposure problem). I think the iPhone relies on your using HDR mode for photos like this, rather than providing an option for metering off the subject. You can also tell the iPhone's autoexposure settings choices are a little strange.
(iPhone: 1/2900 sec, f2.4, ISO 64, pattern metering, AWB; 100 HS: 1/400 sec, f8, ISO 125, evaluative metering, AWB)
Neither camera handles moving subjects very well. The Canon chooses a faster shutter speed, but it has to use a higher ISO sensitivity which results in serious noise-reduction artifacts. It also has an overly cool white balance in the shade. The iPhone, on the other hand, doesn't choose a high enough ISO sensitivity and thus uses a too-slow shutter speed, so the subject is blurred. Its shade white balance is more pleasing, but still a bit off.
(iPhone: 1/60 sec, f2.4, ISO 64; 100 HS: 1/100 sec, f2.8, ISO 200, evaluative metering, AWB)
The iPhone 4S' lens displays a little pincushion distortion (inward bowing), which is odd given its 35mm-equivalent focal length. The 100 HS's barrel distortion is more typical.
I find this the most revealing photo for the comparison for a host of reasons. First, you can see that the iPhone applies strong sharpening to the photos, which not only makes them look a bit crunchy at actual size, but makes it appear as if the color is dithered or posterized. You can also how despite the large maximum aperture specification, f2.4 vs. f2.8 for the 100 HS, the camera's larger sensor results in a more pleasing shallow depth-of-field (the out-of-focus areas behind and in front of the subject.)
(iPhone: 1/60 sec, f2.4, ISO 64, spot metering, AWB; 100 HS: 1/60 sec, f2.8, ISO 100, evaluative metering, AWB)
This shot highlights some of my nitpicks with both cameras. The iPhone shot is sharp, mostly because it chose a low ISO sensitivity, and thus doesn't suffer from the noise-suppression-induced blur that the 100 HS has. It also helps that the iPhone's autofocus is way more intelligent than the 100 HS' and didn't choose to focus on his shoulder rather than his face. On the other hand, the iPhone's lens is quite subject to fringing and flare. That pink tint you see on the right side of the iPhone shot is lens flare. Notice how clean the same area looks on the 100 HS shot.
(iPhone: 1/120 sec, f2.4, ISO 80, pattern metering, AWB; 100 HS: 1/250 sec, f2.8, ISO 320, evaluative metering, AWB)