Coming from Dropbox, the different sharing concept used by UbuntuOne wasn't immediately apparent. (Although I think I like it. Have to play some more though.) Currently if you try to drop a file into the "Shared With Me" folder in Nautilus, you get a standard "Permission Denied" error pop up. I'm not sure how you would go about implementing it, but there should be an UbuntuOne specific error when you do this. Something that explains why permission is denied and how the "Shared With Me" folder is supposed to work.
Error while copying "foo".
There was an error copying the file into /home/andrew/Ubuntu One/Shared With Me.
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Error opening file '/home/andrew/Ubuntu One/Shared With Me/foo': Permission denied