I realise this thread started with reference to the B737-800, however Airbus has recently provided some timely information on the subject of JAR/FAR 25 certification of yaw control.
Basically, the most severe 'in service' encounter of lateral gusts, rolling maneouvre and asymmetrical engine failure must be taken into account. The aircraft structure must be able to sustain a design limit load of 1.5 without permanent deformation or failure over a period of 3 seconds.
There are further requirements/ considerations....