iPad 2's A5 has only 512MB, and so does the iPhone 4S' A5.
With every new generation of a device, we expect improvement. At the very least, we want to see a faster processor with more memory. This is no different between new computers and even new iPhones.
With the iPhone 4S, (though some were expecting a beefed up iPhone 5) we get a much faster CPU and GPU, but what about RAM? Most new top-end smartphones these days pack 1GB, but Apple made no mention of any RAM bump in the iPhone 4S.
Thanks to a teardown at iFixit, we now know why Apple didn't boast about any upgrade in RAM – because there wasn't one. AnandTech examined the chip markings and deduced that it's the same A5 found in the iPad with its 512MB of RAM. This isn't surprising, and it makes sense, from a production and supply point of view.
Developers making apps will also have to consider the lowest common denominator, which for iOS is the iPhone 3GS and its 256MB of RAM. The original iPad also only has 256MB.
Finally, here is a die shot of the A5 from Canada-based Chipworks. It looks a lot different from the image Apple used in its iPhone 4S video.