The extra Mhz here is wasted because Apple intentionally values the heat/battery dynamics more in this product. That's how the whole 1.7 Ghz lines of the Core Duo (and now the Core i5/i7) chips came about: Apple worked with intel to make the chips and got a good deal of exclusivity. This was all made clear when the Macbook Air debuted with Paul Otellini and Jobs onstage together.
EDIT: I misinterpreted the above comment. It's Friday and I'm sleepy.