Rather than thinking of your data being IN the cloud, think of it moving through the cloud, from one computer/device to another.
If at any time apple's servers get totally wiped, your machines will just push the data back to them and will resume as usual. Your information continues to be on all the machines synced. Except for your idisk. That's the only part of it you will need to keep a copy of. I'm surprised the mac doesn't cache a copy of that data locally on each machine. (maybe there's software to do just that?)
