This is from a MobileMe page, so it may not fully apply, but it suggests you'll be able to use separate accounts for iCloud and iTunes.
If I use different accounts for iTunes and MobileMe, can I merge them into a single account and use it with iCloud?
No. You cannot merge two accounts into one. However, you will be able to move your MobileMe account (firstname.lastname@example.org) to iCloud and, if you choose, you can continue to use a different iTunes account for store purchases and iTunes in the Cloud.
I unfortunately have the reverse problem. My iTunes account is actually my mobile me address. I would like to keep that email address and move it to icloud but still be able to share iTunes store content within my family. Hopefully I can just sign into the iTunes store with that id and have all oher features run of the icloud id which would be the same but hopefully with different passwords. As roamingnome said, we will know more when it launches. I was hoping that someone beta testing iOS 5 had the answer.
Thanks for the link above