I think you've found a bug. The choice to Cascade the windows still appears in the Windows menu, but doesn't appear to do anything. You can report it here: https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform
The issue seems to be much deeper than simply the Window/Cascade menu option. The whole UI is stuck in this tabbed interface, which I find largely unusable. They made it extremely difficult to get out of it, too.
I can't work in a tabbed interface. I use as many as 10 document windows open simultaneously, usually some tiled (keeping 2 or 3 always visible) while others are cascading and may be only partly visible. Tiling this many documents within a single tabbed window is horrible, as there is no control over the size or shape of individual documents. If I manage to create multiple document windows I can't arrange them on the screen the way I want. CS5 keeps "helping" by guessing where I want them and there is no apparent way to stop it auto-arranging and maximizing windows again. This just sucks. I can't believe I'm the only person who works this way. CS3 is so beautifully obedient. CS5 is a nightmare--or is there actually a way to make it behave like CS3 that I have not yet been able to figure out?
This UI design was apparently intentional, as was abandoning a NATIVE menubar. This is not progress, except perhaps for the Adobe developers, as I'll bet this UI is easier for them to maintain. Regardless, I reported it as a bug but don't expect anything to come of it. :-(