But in the real world, can these two smart-phone powerhouses back up their battery boasts? We took two brand-new iPhones into our sterile testing lab housed in a geodesic dome under Stonehenge -- okay, a desk nearby -- and turned up the heat.
First, we cranked the screen brightness up to maximum, turned off the sleep function and turned on Wi-Fi so the phones would both be bleeding battery during the test.
We charged both phones to maximum, and set them two tasks -- the longest call ever made, and a whole day of brain-boggling Glastonbury reruns.
3G calling battery test
To test voice calling over 3G, we rang each phone from a landline hooked up to BBC Radio 4, which provided the inexhaustible chat.
The iPhone 3GS managed 3 hours and 31 minutes of Gardeners' Question Time until it was totally wilted. The iPhone 4 ploughed on with the green-fingered chat for 4 hours and 53 minutes.
That puts the iPhone 4 a lead of 82 minutes, an increase of 39 per cent.
Streaming video battery test
In our second test, we connected the phones to the CNET UK Wi-Fi network and let rip with streaming video. We bobbed our heads to the highlights of the Glastonbury festival until the phones were stone dead.
The iPhone 4 managed to squeeze in 7 hours and 7 minutes of Tom Jones' poptastic posturing, while the iPhone 3GS dealt out 5 hours and 56 minutes of Welsh warbling.
That means the iPhone 4 beat the iPhone 3GS by 71 minutes, adding 20 per cent more iPlayer goodness.
Let's compare these values with Apple's estimates for battery life, shown in a hand-crafted table.
Keeping in mind that we had the screen brightness turned up to max, as well as everything else we could do to put pressure on the iPhone's battery, maybe it's no surprise that neither of our phones came close to the maximum talk time Apple's touting. Streaming video over Wi-Fi also missed the maximum mark.
But the iPhone 4 did trounce the iPhone 3GS in both our tests, with 20 per cent more streaming and almost 40 per cent more chatting time. That's not quite as impressive as when we tested the iPhone 3GS battery versus the iPhone 3G, when the iPhone 3GS pumped it up by more than 45 per cent.
Our American cousins at CNET.com are also testing the phones, and they found their samples actually exceeded Apple's talk-time estimates. However, they test with the backlight at 50 per cent and the screen set to turn off after only 10 seconds.
Like most smart phones, the iPhone 4 will still need to be charged every day if you're using it extensively -- but at least it's noticeably better than previous iPhones. And if you turn off some of the good stuff, it can be even better than Apple claims.