Gizmodo reader Erica wrote Jobs an e-mail asking why older version of the iPhone didn't have this option. "See, I get why you don't include multitasking," she wrote, "but the background thing, I don't see how that would be memory intensive and/or battery draining. It doesn't seem like that feature needs to be exclusive to the 3GS and 4G."
Steve's answer was simple: "The icon animation with backgrounds didn't perform well enough."
From my experience on an iPhone 3GS, sliding through screens can sometimes feel a bit twitchy, and (at least subjectively) slower than before. It's quite possible that the icon animations combined with the new backgrounds were simply too much for the poor old iPhone 3G. It seems that Apple really does update the hardware on these devices!