My guess is that they are either:
a) Doing the broad strokes. Laying out their towns/dungeons/forests/etc. in plain old graph paper and concocting the NPCs and their traits/personalities like they would for a pencil and paper campaign. This is probably the most likely since the 3e rules are already known, and in fact are freely available (http://www.opengamingfoundation.org
b) Building tilesets/character models in a 3d gfx program and hoping they'll be able to export it to the NWN file format.