Well, that's a nice feature, but as you bring it up, deep linking isn't possible ;-) There are various techniques to enable it though, pushState(), hash fragments etc.

Best would be a replaceState: would not break back button, would allow deep linking.

The thing I was most impressed by was the fact that the left and right cursor buttons worked straight off the bat :D

I believe it is a side-effect of the keyboard navigation. It appears to support the back button (due to the script listening for the arrow keys and disregarding the Alt-modifier) but only for those users who use the keyboard to navigate back-and-forth in history. If you observe carefully, the actual "back button" isn't activated (and hence useless).