Apple have said this on the developers site
"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."
Yes I've read that a number of times and I'm not entirely sure what it means. I don't find the wording particularly clear.
Does it mean that you upgrade your iTunes ID and that becomes your 'iCloud' account, but you can move *just* your MobileMe mail address to it (all iCloud accounts need a @me.com address right)? Or does it mean that you 'upgrade' your MobileMe account to become your one, true iCloud account, but there is a separate notion of 'iTunes in the cloud', which doesn't use your iCloud id but your old iTunes id and you just keep iTunes using that old thing and don't worry about it.