I installed Jaffa's Horizon in a Maemo 5 unit with accelerometer and it does show something green like a field of grass and something blue like a sky. It moves as you move the device.

The vertical axis seems to work well: pointing the device up/down/middle shows an horizon as expected. I'd say it misses some callibration based on the result when putting the device on the wall but maybe that's not even the app's fault.

But there is something wrong with the rest: when putting the device in portrait mode the horizon is just upside down. This means that your "spirit level" is mostly unbalanced all the time. Wrong math? Psychological terrorism?

By the way, there is already an application called Horizon and it was even ported to Maemo in the past.