How do you hold it?? :)
You can, of course, get your little finger under it move the phone body up so it's easy to swipe with your thumb however that's a lot of hand movement and you need to look at what you are doing.
If you are holding the phone whilst walking you generally hold it as in the image above which is much more secure it would be nice to be able to answer it without having to shuffle the phone with your pinky.
To be honest even moving the thumb horizontally to me doesn't feel like the most natural way to move your thumb. Instead consider moving your thumb from the top left arcing across (i.e. from a 12 o'clock position to almost 3 o'clock) of course you'd need to consider lefties but it feels a lot more natural...
Can we try and not make up stuff?
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The location of the slide, I guess, is to allow people to see their lock screen wallpaper. That's my stab at it, but who knows.
[1]: http://developer.apple.com/library/ios/#documentation/UserEx...