Engadget‘s Ryan Block, by Apple’s invitation, was able to spend a few minutes with an iPhone 3G. I’m linking to this both because I think Ryan’s notes are interesting, and because I’m surprised Apple had people see them when even Steve Jobs didn’t have one on-stage during his Keynote.
Here are some highlights from Ryan’s notes.
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Bandwidth tests confirmed about an 2x speed-increase. 
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You can disable 3G (presumably to preserve battery life). 
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Wifi is 802.11b/g but not N. 
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Geotagging photos is possible. (hooray!) 
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The dock is now sold separately. Docks from first-gen iPhones can not be used. 
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You can use any 3.5mm headphone! 
Overall, I’d say a great update. I’m disappointed that I’ll have to fork over $29 for a new dock, and that 802.11n wasn’t built-in. Besides that, the update looks great.
Source: iPhone 3G hands-on.