A little program called N9Tweak lets you hack your Nokia N9 to enable landscape mode and a few other customizations, like changing the behavior of the power key or resizing the virtual keyboard. From the video, the landscape tweak appears to work across the Applications, Events, Open Applications, and Lock screens and responds to the accelerometer when you rotate the phone. You can download N9Tweak from your phone's browser, after which you can activate landscape by following on-screen instructions — just make sure to activate developer mode and enable "allow other sources" before you begin. Go to the Maemo Forums for a full set of available tweaks and the latest updates. When you're all done fiddling, you'll be able to experience the bittersweet joy of Harmattan all over again.
N9Tweak for Nokia N9 enables landscape mode, other customizations
N9Tweak for Nokia N9 enables landscape mode, other customizations/
N9Tweak enables landscape mode and other customizations for the Nokia N9