For example, on my home iMac, there are three user accounts (my own, my wife's, and our eldest son's) and the computer is authorized with two Apple IDs (my own and my wife's). We can currently share apps, manually, by copying the app file to shared disk and double-clicking them to add to iTunes. All three of us have an iOS device (iPhone 3GS for me and the Mrs., iPod Touch 3G for the eldest) synced to that machine.
Now, with the App Store update as of yesterday, it does appear that I can see my wife's purchases as well as my own, and I was able to copy one of those apps directly to my iPhone, and what's more, using my Apple ID, not hers, to authorize the download.
So it seems that the iCloud might be tied to all devices that sync with each other, not by user or Apple ID.
I note that OS X Lion also has a low-power (CPU/disk only, no monitor or USB) wake on LAN feature for file transfers, etc. -- in conjunction with the iOS wireless sync, I'm curious to see how the sync will work given that each user logs into iTunes separately via their computer account. (Also, in theory, shouldn't the low-power access work in conjunction with being an iTunes client that Apple TV requires to play files from a network disk?)
I think I may have just realized the answer to my own question. Why on earth am I insisting on putting the computer into the equation? The iDevice will sync to the cloud, not the computer. The computer will be updated from the cloud when it gets the chance (assuming, say, that it's powered down at the time of the iDevice sync.)
Ok, correcting myself here. I was not able to see the Mrs.’ downloads - she had gotten the same apps (free) for my son’s iPod, which is linked to my Apple ID.