I've been intrigued by iCloud, especially as an easy way to synchronize calendars across multiple devices, used by my wife and I. One of the things I was concerned about though, was that from everything I was reading iCloud is keyed from your Apple ID. I'd found one forum reference to using a mobile.me email address but that appears to no longer be the case.
Just in case others are in a similar situation, I thought I'd post the info I got from our AT&T store manager today. (Yes, I'm a sucker and we got our 4S upgrades today. But we were upgrading from the 3G - it's long overdue.
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My wife and I each have an iPhone and an iPad; naturally (as I think most families do) we use a single Apple ID so we're not purchasing multiple copies of the same application. Great for apps, not so great for the cloud - if we fully enable iCloud syncing we will end up synching our devices to each other - anything one of us does on one device would get replicated to the other three.
I asked the AT&T store employee about this, to see if there is a way around it. He happened to be the manager, and he had called the manager of the local Apple store because he has the same issue. It turns out that at least for this iteration of iCloud there is no way around this problem - there is no way to differentiate your devices as being used by unique users. This stinks - I wonder if Apple is trying to force families to have multiple Apple IDs, so they end up spending more in the store.
Hopefully this saves someone some Google searching - iCloud information is a little scarce and confusing at the moment, since it's been in beta and changing a lot over the past year. Although if you DO find a good workaround or fix, I'd love to know about it. This problem basically makes icloud useless to us.