Apple released information on the iPhone battery's warranty today, as well as the cost of a replacement.
Out of the box, your iPhone's battery is covered for 1 year. Apple intends to release Apple Care for iPhone sometime this summer (allegedly). Customers of Apple Care for iPhone will be entitled to a new battery IF the one they're using drops to below 50% of capacity before their Apple Care runs out.
Otherwise, a new one will cost you $79US. One other note: Apple states that they get rid of dead batteries in "...an environmentally-friendly manner." So don't get all excited, Greenpeace.