iPhone 3G 3.1.2 with bad battery life.

Hey,
I currently have a 8GB iPhone 3G 3.1.2 and the battery life has greatly decreased from when I first got it. The phone is only about 6 months old so it is still under warranty.
If I make an apportionment with the Genius Bar are they likely to replace this phone or install a new battery? Or is this completely out of the warranty T&C.
Windows 7 64-bit. Intel Core 2 Duo E7400, 1TB HDD, 4.00GB Memory, 2 22" Inch Screens.