Never mind, it was just being very slow. In the end it woke up. The next two times switching it on, it displayed the "Committing database" notification. Perhaps it was just doing some initialisation after the upgrade? It might be useful to notify the user that this is happening - new users especially may get nervous if their newly-Rockboxed player doesn't boot up instantly. (Of course, seasoned 'boxers like me have more trust in the development team!)