Interestingly, Tweetbot/Netbot are the only third-party apps I have whose iCloud features work. Nothing with document syncing or Core Data syncing works. @pth do you have any insight into why that might be? It would help me figure this out.
@aw When I was trying to debug my iCloud problems with Apple, I tested Reading List and tab syncing. Don't work. Totally busted. I don't use Safari, but now I have no reason to, even though Chrome is awful on Mountain Lion.