Mac(Book)s (and Macs with the magic apple mouse) have two finger scroll which is the same thing, but isn't supported (but should be, please!) on say the headline box in the Skimmer "Spread" layout.

They are almost the same thing. They perform the same function but because they are physical invisible they are even less likely to be used by an average computer user.

I have some anecdotal evidence on the contrary. My wife is a very casual computer user, but uses two-finger scroll all the time. My sister-in-law is very similar - not a computer person, but has an Apple laptop and uses two-finger scroll. Also, Apple mice support scrolling[1], and almost any third-party mouse you buy will have a scroll button as well. Scrolling with a mouse or trackpad seems like an integral part of user interaction.

[1] http://store.apple.com/us/product/MB829LL/A?fnode=MTY1NDA1Mg...

And every Mac user I know is well aware that for the web, <space> scrolls a page forward, <shift-space> a page back. So still no good argument for on-page controls.

Counterpoint: I didn't find out about shift-space until just now.

Counter-counterpoint: I didn't know you until just now :)