Basically the layout would not load when I rotated the tablet while the application was running.
If I started the application in portrait then that layout was correctly selected and when I started the app in landscape that layout was likewise correctly loaded. But after one layout was loaded the orientation change did not trigger Android to switch automatically.
The culprit was in my AndroidManifest.xml file. For some reason when I first created my activity the configChanges attribute had been set automatically for me to:
android:configChanges="keyboardHidden|orientation|screenSize"
removing screenSize solved the issue:
android:configChanges="keyboardHidden|orientation"
Obvious eh...
Edit: This behavior is actually documented in the android development guide.