Important information for those with both @domain.com and @me.com accounts
I'm in the same boat, and I won't be able to try this exactly until I'm off of work tonight. Good luck to us both. If you find an answer I'd appreicate a reply or PM letting me know how it went! Thanks.
So I am one of those people who have an @me.com email address for MobileMe with contacts, calendars, etc. and a @domain.com apple ID for iTunes purchases.
Since I don't use the @me.com email address, instead of moving my @me.com address to iCloud and keeping 2 accounts I decided to set up iCloud with my @domain.com apple ID and only have one, making things easier in the future.
This worked easily enough on my Mac, although I did need to sign out of MobileMe to enable some of the features (photo stream, bookmarks sync). I then told my computer to sync everything with iCloud (calendars, contacts, bookmarks, photo stream, documents, back to my mac, find my mac). For everything except mail and calendar this happened seamlessly without me having to do everything. It is all there on iCloud.
For mail, I didn't set up an email address, because I'm still hoping one day to be able to merge my @me.com address into this account. So I just leave that option un-ticked for now, if there is no change by the time MobileMe shuts down I'll start a new @me.com address then.
For calendars, each of my calendars had to be manually transferred from MobileMe to iCloud. So I set up a duplicate calendar along with its corresponding reminder on iCloud. Then I exported each MobileMe calendar seperately to the desktop, and imported them as the corresponding iCloud calendar and reminder. Voila everything transferred successfully!
Still having issues getting my iPhone updated to iOS5 though!
Let me know if anything is unclear or you have further questions. I'm happy to help