It's been confirmed that the iPhone 5 has 1GB of 1066MHz RAM, reports AnandTech.
Apple thankfully didn't obscure the details of its A6 slide at the launch event, which gave us a Samsung part number: K3PE7E700F-XGC2. Through crafty navigation of Samsung's product guide, Brian Klug got us the details. The K3P tells us we're looking at a dual-channel LPDDR2 package with 32-bit channels. The E7E7 gives us the density of each of the two DRAM die (512MB per die, 1GB total). The final two characters in the part number give us the cycle time/data rate, which in this case is 1066MHz.
According to the site's calculations that gives about 33% more peak memory bandwidth than the iPhone 4S which can be used to feed the faster GPU and drive the larger display.
Take a look at the chart below...