What is the root cause of famous iPhone 4s battery draining issue? Even with latest iOS 5.0.1 some 4s users do encounter huge battery drains (1-2%/minute) even when they are doing nothing with the phone.

Is it a hardware or a software issue? Should I get a refund?

Update: The phone was bought 3 days ago from Apple Store in London, UK.

Last night when the phone battery went to 10% the phone just shut down automatically, like other were doing when the battery was reaching 2%.

Also, I observed that the percent of battery discharge was not linear, meaning that sometimes I observed the batter level going down with 5-8% at a time (while using the Phone). If I remember well from the other iPhones I had, the battery discharge in percentage should be smooth.
If it's convenient, get a Genius Bar appointment and take it to an Apple store. I have heard that further software updates to correct battery drain problems will eventually be released. However, you have every right to be unsatisfied with the product in the condition it is in right now.