Apple has updated its App Store Review Guidelines web page with a new rule that specifically bans any Apple Watch apps that just tell time.
This effort would seem to be an attempt by Apple to keep any third-party custom watch faces from its Apple Watch device. The new rule is under section 10, "User Interface" of the App Store Review Guidelines. The specific addition is 10.7, which says:
Watch Apps whose primary function is telling time will be rejected
At the moment, the Apple Watch just has 10 watch faces, most of which can also be customized to some extent. There's no word on when, or even if, Apple will eventually allow the smartwatch to support third-party faces, but it seems likely that the company will release some additional official watch faces at some point.