My iPhone 4S cellular data usage stats from October 22, 2011 to September 22, 2012.
I used 11.4 GB of data in 336 days, or about 34 MB/day. For comparison, I used about 14 MB/day on my iPhone 4.
This continues the trend of increasing data usage with each new device, but the jump from the 4 to the 4S is the largest jump I’ve seen so far (2.4x increase in daily usage). I would attribute the large increase do to my the amount of traveling I did this year. Without WiFi, I relied on my phone quite a bit, especially in countries where tethering was allowed.