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Maybe it's for the same reason so many digital clocks come with a fake display sticker that reads 12:08. Perhaps it just looks good to whatever art director was in charge of the photo shoot.
 
it's supposedly because Steve announced the first iPhone at 9:42 at MacWorld.

Yeah. There was some discrepancy over the issue of exactly when he was supposed to announce it. I think one of the first iPhone ads had 9:41 displayed, because that was when he was "supposed" to announce it.
 
I don't think this is that obsessive. Apple tend to pay attention to these details. The old iCal icon defaulted to 17 July because that was its launch date, for example.
 
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