Many owners of the iPhone 4S are complaining about very short battery life, with their fancy new smartphones sucking down the juice like it's going out of style. The good news is that Apple is aware of the problem. The bad news is that they have no idea what's causing it.
Apple engineers have been getting in touch with people who have reported the battery issue, asking them for details and data from their phone and having them install a diagnostic file that sends extra info back to Cupertino. But apparently the engineers there are still pretty stumped as to what's causing it.
Is it a reason to not buy the phone? Not really, as I'm sure they'll figure it out and fix it before too long. Just carry a charger around with you for the time being.