My post after the iCloud announcement got a lot of comments. Today Apple answered the question in their MobileMe transition FAQ post.

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 (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.

Works for me.