Before I got my iPhone 4 I downloaded the true HDR app for my 3GS. It’s normally about a dollar, and gives you a decent amount of control on shooting HDR photos with your iPhone/iPod Touch. It will work with any iPhone that has a camera.
This is an image from the same location shot with True HDR. This was done with the auto capture mode, but you can shoot images with more control if you want.
Both were taken on the same rainy day in the Opryland Hotel in Nashville, TN. As you can see, the HDR image doesn’t have the blown out sky. It works great for landscapes.
In fact, it’s almost only good for wide shots. The nature of HDR means that you cannot have movement between the shots. The pictures won’t merge correctly if you do.
But, for the right kind of shots, True HDR gives you more control than the native HDR function on the iPhone 4, and allows 3GS uses and iPod Touch users to shoot high dynamic range pictures.