You can take it in, and they'll most likely either replace it or else test it first and then replace it as necessary. The battery should easily last several years for daily use, and the amount of battery time you're describing is a lot less than expected (assuming you're bundling in the time you spend with apps, games, etc, in your usage time -- games and apps drain the battery probably faster than almost anything).
On the other hand, there are some other things that rarely are big deals -- if you have very weak signal in your area and your iPhone keeps going in and out of signal reception, the battery wears down very quickly (true of essentially all phones) in this scenario.