Thanks to Anthony for getting this response from Yamaha:
At this point in time we have no official release date or estimated time of arrival for the Tenori-On iOS app to hit the US Apple store. Information will be released as it becomes available and will be posted on one or both of the following sites:
PAC Division Applications Specialist
Yamaha Corporation of America
6600 Orangethorpe Ave.
Buena Park CA 90620