I upgraded to Mountain Lion yesterday and wanted to sync my iCloud mail and notes. The problem is, that i can't activate the checkmark in the system menu for notes and mail. Activating on the iPhone worked perfectly. Everytime i want to check the boxes for notes and mail it occurs an error: The application "Notes" cannot be activated at the moment - Please wait and try again activating "notes" same error for Mail.. I already created an @icloud.com-Account and verified it. Mailing over the iCloud.com-Account also works! I just can't activate Notes and Mail in the system menu.. (Sorry for bad english )
Then my bet would be to wait for 10.8.2. Also ask in the iOS developer forums, I have not extended my developer account, therefore I can't dig into that.
If you also have a @me.com address give that one a try. It will work.