Apple has posted an in-depth page outlining the transition to iCloud.
This part resolves a concern raised by many, including me:
You cannot merge two accounts into one. However, you will be able to move your MobileMe account (yourname@me.com) to iCloud and, if you choose, you can continue to use a different iTunes account for store purchases and iTunes in the Cloud.
Additionally, family account members can migrate their accounts separately — good news for those of us with multiple AppleIDs in the household.
Oh, and mail, contacts, calendars and “Find my iPhone” will continue to exist as web apps.