One of the early concerns about the iPhone was battery life with the non-replaceable battery. While most of those concerns seem to have been allayed, I'm sure there are still folks out there that run their iPhones all the way down on a regular basis. For those people, the Mophie Juice Pack
may be just what the doctor ordered. This external battery is like a slip case for the iPhone adding "up to [an] additional 8 hours" of talk time. The Juice Pack is itself charged by a Dock connector and intelligently supplies power to the iPhone so that the Juice Pack completely runs down itself before letting the iPhone switch to its internal battery.
The Mophie Juice Pack is available for pre-order
at $99.95 with an estimated ship date of Dec 21.