Well, it works in 5.3.7 firmware!
It is also included in the Mesquite API (check in /var/local/mesquite/shared/javascripts/device.js).
Code:
kindle.dev.setOrientation(orientation);
Orientation variable can be:
Code:
orientations = {
auto: 'auto',
portrait: 'portrait',
portraitUp: 'portraitUp',
portraitDown: 'portraitDown',
landscape: 'landscape',
landscapeLeft: 'landscapeLeft',
landscapeRight: 'landscapeRight'
};