AppleInsider | Apple's iPhone 3G battery good for about 3.5 hours of browsingI've found that the battery life on my low end 3G Nokia phone is similarly quite poor, even though that phone has no data services at all. 3G is bad on batteries even when the traffic is voice only; of course 802.11 is far worse.
...While Anandtech's chart shows the Apple handset to last about 30 minutes more than Samsung's 3G Blackjack, the unsettling comparison exists between the iPhone 3G running on AT&T's 3G network and the original iPhone running on AT&T's EDGE network. In the site's tests, the original iPhone lasted 2 hours and 26 minutes longer while browsing over EDGE than the new iPhone did browsing over 3G...
We know the iPhone will function on EDGE networks if 3G is not available, I wonder if Apple and AT&T will provide an option to use EDGE when power is at a premium. (See comments -- in fact this is available now. So why do only my readers know to mention this?)
Meanwhile, a solid Tidbits review recommends the APC UPB10 Mobile Power Pack USB Battery Extender. These are $63 on Amazon; however there are newish airline regulations about carrying external LiOn batteries. The terminals must be covered and it has to go through security in a bin.
These regs are enough of a pain to make the $10 APC external USB AA battery charger very appealing; I would want to test in a store that it really charged an iPhone however.
At work it's easy to charge a phone from a USB source, on the road it's easy to carry a USB car charger. One does have to learn to treat a power supply the way long distance cyclists treat a shower -- get it while you can.
Update: disabling push email helps too, Ars has a in-depth review of accessory battery packs.
Update 7/23/08: More tips, including holding down home key to shut off background tasks.