I just installed Mountain Lion and Pages 4.2. In the open dialog for pages with iCloud highlighted, I dragged a pages doc to the dialog box. It looks like it works since there is now an icon in the Pages dialog box with the correct file name. BUT it has a little cloud icon and "Waiting" under the document icon. I can open this document on my iPad, but I can NOT open it in Pages on my iMac. OS is 10.8.
It turns out it was a corrupt Mobile Documents folder in the library. The genius at the Apple store figured it out after much head scratching. Moved it to the desktop to force a new one to be built in Library and the problem was solved. Apparently, I was lucky enough to be the first guy to hit it. :-) No idea how it happened though.