Your iPhone as a DSLR

There is no way, even with the new iPhone 4S, that you will be able to get as good a picture with an iPhone as you can with a digital SLR camera. There's also no way to set aperture on an iPhone to get a narrower depth of field. Fortunately, there is an app that gives you the ability to give this appearance to your iPhone photos after the fact.

I've used several "paint-on blur effect" apps in the past but Big Lens seems to work the best. With this app you can paint the part of the photo you want to be blurry or clear with your finger and then set the aperture for the blurry part. You can pinch to zoom to do more precise "painting" and the app even has an "auto" option that will attempt to determine what you are trying to make clear and paint that for you.

I had mixed results with the auto option. It seemed to work better, the less busy the picture. However, by manually painting on the object in focus, I was able to get a really good focal effect. It's important to note that the amount of blur is based on setting an aperture setting as you would when taking a picture with a DSLR.
The app also has several bokeh effects that allow you to choose shapes for out-of-focus points of light.  This can create some nice looks. In addition to the bokeh effects, there are a number of filters that can be used to add a little something extra to your photos. Most of the photos are nothing special that you can't find in other apps like Magic Hour. However, the flare effects, which I haven't seen in any other apps, are pretty nice.