If the iPhone is anything like the iPod (I'm almost certain it is) In order to replace the battery, you need to send it in to have it serviced. In this event, after they receive your phone, they will send you a refurbished one, fix the phone you sent in, then give it out or sell it as refurbished.
Was interested in replacing the bat in my iPod before I broke it, so I looked it up.
I'm not exactly sure of this, but with the way Apple seems to completely seal everything off to people who don't have tools, it seems even taking off the cover might void the warranty.
Just my 2 cents. Great.... Now I need to work 2 days of over time because of posting...