AppVV confirms that the iPhone 4S has 512MB of RAM. In a discussion thread an administrator reports that it has 512MB. This fact is repeated on their Weibo page as well in response to a question:
Re @ LogIn_Kim: RAM have said before, is still 512MB.
Though they don’t detail how they came to the figure, we’re inclined to believe AppVV as they are the ones who posted a hands-on video of the iPhone 4S.