It's pretty standard for top-tier smartphones to come with 1GB of RAM these days -- a lot of people were expecting the same of the iPhone 4S. There's no indication that the phone suffers performance-wise.
iFixit deduced that the iPhone 4S only had 512MB of RAM based on markings on the A5 processor. Anandtech has done some digging of their own based on iFixit's images and determined that the Samsung part number on the processor indicates that the phone indeed carries 512MB of RAM. Check out our previous post for a full rundown of what we learned from the iFixit teardown. [iFixit and Anandtech]