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If they changed the back to be all plastic, they must have changed the way it's held on. The screws are probably just the outcome of that.

If Apple were going to go-to the trouble of making the battery user replaceable, they would have gone to the trouble of making it user replaceable in a user friendly way. Screw are not exactly user friendly, not for changing the battery of a mobile phone anyway.
 
I'm not seeing this as a battery thing. (and I don't care about that owning the 1st gen for a year).... I see it as a way to remove the back cover for techs (us). I'm envisioning being able to buy a new back from Apple to replace a cracked back fairly easily. I'm envisioning being able to change the color.
 
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