I assumed you'd already have the source Finder window open. We always move, make aliases or copy files using two Finder windows. It's just so intuitive.Bingo! That's it! But you left out a step. After opening the open-document window you have to go back to Finder, open the folder that contains the files you want to move and then Drag&Drop those files to the open-document window which has the designated iCloud option open.
A contextual menu item might be nifty. However, I imagine that lots of people might move files to iCloud and then think they're lost.
Would I lead you astray? As I mentioned above, Mountain Lion keeps a "secret" copy of the files. (I tore myself away from a toilet-plumbing project to find these links just for you. Or is that too much information? )Just another observation. After moving the Pages and Numbers files to the iCloud they no long appeared in the Finder on my iMac. So I'm guessing that you don't have duplicate copies on your Mac and the iCloud, just one copy in the designated location.
http://hints.macworld.com/article.php?s ... 3092156975
Note that this gives you another hint for moving multiple files to iCloud.iCloud documents are cached on your local machine so that you can open them even if you don't have Internet access. The files can be accessed in the Finder in addition to the Apple application dialog boxes.
You can access them at this location:
Each application has its own folder that contains its documents. Files can be added or removed from this window.
Because Time Machine is context sensitive, if you have an iCloud window open, then enter Time Machine, it'll let you browse back in time just for iCloud. (This might be application specific.)
This doesn't address the potential problem of having two unsynced files with the same name in different folders. But that's always a danger, with or without iCloud. I recommend assiduous file duplicating and naming.