Does not work on Lion 10.7.3 and above. The problem may be solved by launching the command
while true; do /System/Library/PrivateFrameworks/Ubiquity.framework/Versions/A/Support/ubd; done
in a terminal window and leave it permanently open ! Not found any other solution.
In few words, create a directory named DropCloud in ~/Library/Mobile Documents on each computer, turns on the documents management in iCloud Prefs, and that's it. So the program is extremely minimal in value. If it had fixed the Lion issue, then it would have been a real improvement. But the developper is very young, and he will certainly have time to think about the ubd process in Lion > 1.7.2 !