OS X Hints show you how to access your iCloud documents via the Finder. The folders inside of Mobile Documents are all named a little strangely, using the bundle identifiers, but you can mostly tell what each folder represents if you read the names carefully enough.
I wanted to add this folder to my Sidebar, but I noticed that Finder wouldn’t let me. There’s an easy enough workaround to do so though; you can make a symbolic link to the Mobile Documents folder and then add the link to your sidebar. I made a symlink in my Home directory called iCloud and then stuck that in the sidebar:
ln -s ~/Library/Mobile\ Documents ~/iCloud
Then drag the symlink to your sidebar.
It should be noted that I greatly prefer standard Unix symbolic/soft links over Finder Aliases because some command line tools have a hard time following Aliases; most specifically,
cd can’t read Aliases for shit. There are workarounds, but I prefer to just dodge that and use symlinks from the get-go.