How difficult is it to do cell shading in nexuiz? I'm pretty sure I've seen it in one of the levels but I have no clue how it works.

It can't really be done, that's why it looks somewhat weird. Just like in W*rs*w and Q3A, cell shading is just done using Q3 shaders, so it is "emulated" using q3map2 generated map geometry. http://www.quake.de/index.php?action=q3map2_celshading&q=q3 (sorry, it is German, but if you can't read that, try using Google and the keywords you find there).

Now I don't know how accurate it is as I do not know German, but using google translate might help you. It's actually pretty good with Japanese/English so it should even be better with normal languages like German

Here's a link: The link

You can search "google translate" to find that site. A direkt link to it is here: http://www.google.com/translate_t

Enjoy.

savagex had a glsl shader once