If the developers of Things.app and NoteBook.app could use that (an iCloud API) to keep data in perfect sync between the iOS versions of their products and the desktop versions, my life would be complete.
I am sure that will be possible - that's exactly what the iCloud is intended for.
Personally, I am hoping for a more generalized "file system" type approach.. where I can access all my iCloud hosted files via desktop file system, irrespective of what app they "belong to". And the same access to those files on the iOS devices via apps like Good Reader or ReaddleDocs. It is unclear whether Apple will permit iCloud use in that fashion.