Missing a spirit level at home, I decided to use the Wiimote as a replacement. I am still working on it (for example the Y axis is a bit too unresponsive), but I thought I'd share what I've got.
Download & extract the archive, and place it in your apps folder on your sd card so the Homebrew Channel can find it.
Put the wiimote on a surface you want to use as a reference and press A.
This will start a calibration, and afterwards you can put it on a target surface to see it's difference in inclination.