Apple is always known to intentionally block features on older devices in order to push people to graduate to newer devices. One such functionality that is available on iPhone 4 but is not provided on the older iPhones is the HDR capability. As you may already know, this functionality delivers effective contrasts on pictures by superimposing three pictures that are taken in normal, under-exposed and over-exposed modes.
Now, there are jailbreak apps that will let you enable this functionality on the older iPhones. From what we’ve heard in the past, the functionality works perfectly on the older models and there is no reason why Apple shouldn’t have offered the feature to the older iPhones as well. Nevertheless, if ever Apple had decided to block this feature on older models, it appears that this could have been a last-minute decision. Folks at 9to5Mac have uncovered a couple of HDR icons from the iPhone SDK that reference to iPhones with retina display and those that do not have this display. The difference can be noted in the nomenclature that usually contain a @2x in case of documents pertaining to the retina display.
If you were to stay an optimist, what this could mean is that Apple could bring the HDR functionality to the older iPhone; iPhone 3GS primarily, via a future software update. The question is however whether Apple would want to.