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Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A538b Safari/419.3
The "1A538b" was also "1A543a" at the iPhone's product launch, according to Matt Cutts' blog. I'm not sure what the two strings are all about, but it's still a specialized instance of Safari. From my research so far, it looks like that's the only browser you can get the iPhone to use currently.