In my family of 5 we have 4 iPhones, 3 iPads and an iPod touch. Everyone has their own AppleID and mobile.me address under a single family plan. We use home sharing to share music and apps.
It seems to me that this model would work well for iCloud as follows:
* Mail, Calendar and Contact syncing and other items backed up by iTunes today (IMs, Photos, etc.) should be tied to AppleID only.
* Content syncing (Music, Videos and Apps) across all devices attached to your AppleID should include items purchased under your AppleID PLUS items you acquired through home sharing.
Thus, you can only download items previously purchased through your AppleID but you can acquire items from other family members AppleIDs through home sharing and, once acquired, they will be synced across all your devices.
Does this sound like a reasonable solution?