I'm having the same issue. Not sure if it has anything to do with the phone upgrade issues my girlfriend and I just had, or not, but I can't delete my previous iphone backup.
My account shows my old iphone backup and my ipad backup. I can't delete the old iphone backup from my new phone, or my laptop (via system preferences).
Any suggestions other than deleting the account and starting over? I would prefer to not do that, assuming there is a better solution.
The girlfriend's account had a previous backup as well, but hers was deleted without issue (although she had issues restoring, i'm sure those were simply server connection issues).