HDR Camera Enabler for iPhone 3G and 3GS with iOS 4.1 Beta is out on Cydia now
To install it, your iPhone 3G or 3GS need to be jailbroken, naturally, so they must be running the custom 4.1 Beta version of Apple's iOS, available through PwnageTool. Head over there if you are feeling hung out to dry regarding your HDR passion.
Apple iPhone 3G Specifications | Review
Apple iPhone 3GS Specifications | Review