The iPhone battery topic is a hot one. You actually cannot replace the battery easily. You would have to send it in to Apple for a fee.

If your iPhone battery fails outside the warranty, Apple charges $79 + $6.95 for shipping to replace it.

In Asia a iPhone battery replacement kit surfaced now for only $20. The kit contains an iPhone battery (1400mAh, 3.7V), tools and an instruction manual.
On the label of the battery it reads: "Replacement for iPhone". That this is not an original Apple iPhone part is a given.
The replacement I think would even require some soldering as the wires are directly connected on the battery contacts.

Right now nobody should need this replacement iPhone battery, as all iPhone are still under warranty. After the warranty expired, maybe this class-action law suit brought down the cost of an iPhone battery replacement.

I would stay away from this iPhone battery kit - available on Brando.