Like one or two other users, I'm interested in using QGIS to map an imaginary flat world. A previous answer suggested simply refraining from defining a CRS for such a world, but I cannot seem to make that work well with QGIS 1.7 on Mac OS X. Pretty much every time I interact with a layer, the software warns me that no CRS is defined, and if I ever inadvertently open the CRS dialog for any reason, QGIS appears to apply the default CRS to my map. That happens quite often when I'm fumbling around looking for a way to find a setting that will establish the appropriate scale for distances on the map.
The fictional world is a flat plane of unknown (and possibly infinite) extent. The map is centered on a location that is locally taken to be the center of the world. Although the world is of unknown extent, the mapped area is about ten thousand kilometers north-to-south, and the same east-to-west.
I can map the world adequately using tools like Photoshop and Illustrator, but obviously they aren't GIS systems. I'd like to use QGIS for the myriad additional data-handling features it would provide, but I'm pretty well stumped by my inability so far to find or create a CRS --or to get QGIS to accept working without one.
I'm close to concluding that I'll have to write my own quasi-GIS system for this purpose that obeys the (admittedly idiosyncratic) rules of the fictional world.