I have this text saved for situations like this. Have posted it on the forums before, and it is mainly about Dropbox, but I think it would also apply to iCloud:
Realmac says you should not save files on Dropbox. They say it could corrupt your file.
I have been doing this for over 4 years now, and have only had one corruption, and it was my fault, not dropbox's. If you are going to do this, you have to be sure you follow these rules:
1. NEVER have the file open on more than one machine at a time.
2. NEVER allow two people to work on the same file at the same time.
3. ALWAYS be sure your computer has synced completely with Dropbox before opening your file, EVEN if what is syncing is NOT associated with your file.
4. When possible, compress your file as a backup, so if something big does happen, you have a backed up version of your file. You may also want a back up off of dropbox for same reason.
I follow these rules for any important files (since I do support for Joe Workman, I am constantly working on different project files on different computers depending on the time of day, so for dumb test files, I might not follow my backup rules, but I am sure on the syncing and only open on one machine. The only issue I ever had was working late one night, I left a project open downstairs, fell asleep working on it, woke up, didn't save it and went to bed. Next morning I started working on the laptop this time and saved it, that killed my project file.
Hope this makes sense and helps you out!
Now, I understand Realmacs reasons for not wanting users to save work on Dropbox, not sure what their stance is on iCloud. They must see lots of files that get corrupted, and it is just easier to tell people to not use it, but if you do it correctly and follow those rules, you should be good to go.