Photo opportunities are often missed because the user has to spend time navigating to the app. iOS 3 had the ability to map the home button double-click to the camera, but that feature is gone in iOS 4. The camera app is on the lock screen. The volume-up button (or the clicker) acts as a shutter release.
Steve Jobs has always had an obsession with removing buttons away from Apple products. This has been the case for the iPods, iMacs and even the new Apple Mighty Mouse. Still not convinced? Look at Steve Jobs' clothes during his presentations - he doesn't wear anything with buttons either
To minimize hardware buttons (simplify UX), to keep focus on software (no hardware keyboard), and the camera is not the main purpose of the device (and initially was not that good).
Of note, some apps (Tap Tap Tap's Camera+, Quick Snap - Camera Plus) tried using the hardware volume control as the shutter button, but got rejected/removed from App Store for violating Apple's guidelines of not using buttons for other intended purposes.
They could eventually add a dedicated camera button like some Android phones, but I find that unlikely.
Prior to iOS4 you could configure the home button to invoke the camera application so there was probably little incentive to provide a dedicated button. iOS 6 made it easy to launch the camera from a locked screen, and mapped the top volume button to the shutter on the camera. iOS 7 lets you use either volume button as a shutter release. A dedicated camera button would break that UX design.
photo credit: Yutaka Tsutano, https://www.flickr.com/photos/ivyfield/4735774499/in/photostream/