I now realize this is a SYSTEMATIC problem -- not a glitch experienced by a few users. Anyone who already had an AppleID that dates back to the days before MobileMe probably has two email addresses set up in some kind of account capacity.
And it never mattered much before, and now it does: In my case, all my MUSIC got purchased on the computer by way of my old iTunes AppleID... but now all my APPS have been purchased with the MobileMe address. (And why not? The app purchases were for iPhone apps before these were linked in any interesting way with desktop apps and integrated services.)
If I want to sign up for iCloud, I apparently need to chose WHICH set of purchases to leave out of the automatic loop. (And my experience with iPad upgrade involved being told that certain apps would be *deleted* because they were associated with something other than my primary account. So it's not just about feeling left out of the new Cloudy Synchrony Joy; it's about wondering whether my devices will disown some of what I've paid for -- if not now, then down the line.)
Furthermore, I have a dilemma: the MUSIC purchases, accumulated over years, are quite expensive. So it would seem rational to consolidate around that older (pre-mac.com) ID, which connects with my long-time primary email address anyway...
Yet the me.com address is bound to be better supported by Apple with future features and better integration with the OS across devices. So perhaps I should convert my iTunes music into unprotected formats and cope with not having that music recognized as a past purchase (though I would still be able to play and copy the files).
I used to feel like I somehow *messed up* in finding myself with two AppleIDs, each with a long list of purchases.
Now I realize that my experience is probably REPRESENTATIVE among long-time users of Apple services.