We'll probably never hear an official justification for why Apple only put 512MB of RAM in the iPhone 4S. But you know what? They don't need one. Because you'll hardly be able to tell. Here's why.
The more RAM you pack in a device, the more power it uses (at least until the next generation of RAM comes along, then it resets and starts all over again). For decades, power consumption was a secondary concern for gadget hardware and software engineers. They had an unlimited supply of energy piped through a cord. But now that battery-powered devices have become our most vital trinkets, how long that device can live without a recharge doesn't just affect performance—in a lot of ways it is performance. And since they jumped into the smartphone pond feet-first, Apple has made battery life a priority. It's why the first iPhone didn't have 3G, and the reason why the iPhone 4S doesn't have 4G. It's why the iPad 2 also has 512MB of RAM—but still beats the pants off of the burlier competition.
And at this point, we should know better than to freak out about raw specs. Apple's hardware performance mantra has always "more out of less." And they put as much time into optimizing their software as they do the hardware itself. iPhone owners posting in the comments of the Anandtech have reported that legacy iPhones running iOS 5 have actually seen performance gains, most likely because iOS 5 was not as much of an overhaul as iOS 4 was. Apple hasn't just added features; they've also had time to trim the fat and become less resource-intensive. Compare that to the near-crippling effect iOS 4 had on the iPhone 3G.
Yes Apple should not get in the habit of complacency and reactionary moves, but the truth is that their hardware is not middling. Android handset makers need the hardware muscle—a gig of memory is standard issue these days for a top-tier Android phone—because Android is not optimized around a narrow slice of hardware like iOS. They have no idea when Google will push new features and what it will do to their devices. They can only rely on raw power to futureproof their devices. Those Android devices need that gig of RAM as a defensive maneuver. For Apple, it would be an unnecessary luxury.
And while it's hardly the main reason we should grant Apple a pass here, the fact is that building the best device for the least amount of money is why Apple can stay lean and focused on high-quality products in the future. Doubling the RAM might not cost that much in any single device. But compounded across millions of devices, that's a lot of money that could be better spent on R&D, especially if that 512MB of RAM is still suitable for another generation (which it seems to be).
So yes, the iPhone 4S has less memory than the next guy. But the smart money says you're not ever going to notice. And that's quite a trick.