The first time I got to the little cube, I went off to the side and got on top of the building you come out of; Then shot a portal across the big cube to the other side.

Ok. The cube bit is a bit confusing, and can be a little troublesome, so I've made a video of me running thru it.
http://www.youtube.com/watch?v=SUO4-hScy3Q
You can probably tell from watching it, that I havent played it in a while, and it took me a few seconds in a couple spots to remember where to go. I also goofed in a few spots, but oh well.

Cheese: in hindsight, instead of a walkthru, perhaps I should have called it a fling-thru?

yeah ok. only problem is that if you do not know WHICH path to take... you will try "anything" and thusly fuck up the cube and get it all cock-eyed and shit. not much you can do about it i think. its just the way the engine was designed. you'd think a 100ton spinning object would have more momentum than a little lady... but nope. that little lady will stop said object