I'm posting this because I want others who run into the same problem to find a solution - I had some difficulty pinpointing why I was having the following difficulties. Hopefully, if you're reading this, my experience will help!
So, I have two iMacs - one is a mid-2011 model, one is the model before. I upgraded both to Mountain Lion with no difficulties. However, yesterday I began tooling around with iCloud on the newer model, and found that while some iCloud features worked - like Notes and Calendar - others did not, specifically, iWork. I have had no problems syncing files between my iPhone, iPad, and other iMac, so I've been able to retrieve Pages / Keynote / Numbers files with no problems. However, with the mid-2011 model, I was getting just an empty iCloud page.
I did the basic stuff - ensured that I was signed into iCloud with the correct account, ensured I had fully-updated software, and the like - but still, I was getting no love from iCloud. I searched a few forums and checked out some ~Library folders, but everything looked correct - everything was mirroring my older iMac's setup.
I initiated a call to Apple and the front-line rep had me create a test admin login to see if there was something wonky with my user file, but the problem persisted. (By the way, talking to a rep on the phone while they initiate a screen sharing program and following their red pointer around was great). The rep quickly bumped me up to a senior person, who investigated more files. And then we hit on the solution, one I didn't think to check out because it was so obvious.
A few months back this iMac wouldn't boot up, and Apple replaced both the power supply and the logic board. However, the folks at the Genius Bar forgot to "re-serialize" my iMac - the computer was nearly fully functional, but didn't have an internal serial number after the logic board swap. It's like a two-minute step that someone forgot to do.
I brought the iMac to the Apple Store today and in two minutes they had re-serialized the iMac. When I got home, everything was working as it should - the computer has its original serial number, and iCloud worked perfectly.
So there you go. If you have some parts of iCloud working, but not others, check that serial number - iCloud uses it for some, but not all, functions. And if you're getting a logic board swap, verify that your iMac has been re-serialized. It'll save you a trip later on.