I just purchased the Nexus 7 and noticed when using it in my car the screen darkens when turned horizontally. This is a bit annoying since the dashboard mount I have for it sits that way. This doesn't occur at any other time except in my car. The only factor I can see that is different is that I have an audio cable attached for sound through my speakers but it still shows the screen fine when held vertically. ???
to combat this go into settings (if this is your problem) and under display click on brightness and uncheck auto brightness