The battery is not welded in place, like the first generation iPhone battery.
From:
http://www.iphonebuzz.com/iphone-3g-dismantled-display-glass-separate-battery-not-soldered-113006.php
Further inside, and it turns out that the iPhone 3G's battery isn't actually soldered in place. That's not quite as useful as a user-replaceable battery, but it does mean that if you're reluctant to send your cellphone off to Apple and pay them $86 to swap it out, you'll not need a soldering iron to do it yourself.
Discussion on:
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