I just accidently bumped the spacebar and Safari scrolled down. I thought "Hmm... I wonder what happens if I press Shift+Spacebar". And sure enough it scrolled up :). Looks like I discovered another thing in Mac, like before I found that you can hold the Apple key and move the Apple default icons around in the menubar.
It's a semi-standard UI convention, actually. I first saw it in Eudora about 5-7 years ago, and I added it to some of my own programs after getting some requests.

Only I don't use shift-space to go back. As the guy who first requested the feature explained to me, using the B key makes it easier to go back and forth, since you can place your hand naturally on the keyboard and not have to move it at all. Anything involving shift is a hassle. But I guess the B behavior is somewhat less standard (and admittedly less intuitive).