I very rarely use radiosity, because of the exponential increase in render times and because it's not needed most of the time. 

http://www.youtube.com/watch?v=XCEN0qQOsIA

Here's a tutorial for blender, it's long, but covers just about everything, and should probably be exportable as an obj, I'm not that far along with Blender that I could try it.  He makes it seem simple, but it's like everything else, simple once you've done it, not so simple before then.

Trying to do this, on line, and trying to watch the chargers on several model helicopters at the same time.  Let them go a little too long and the batteries let the smoke out and stop working.  Electrical things operate on smoke, that's why we don't want to let it out.

I'll have a play with Citygen a little later, it's possible it is exporting a mesh, although I would have some doubts there, I just don't remember anymore.  I don't work much with cities, but more woods type stuff, small areas.  I ain't no city boy, don't even like small cities.

The "I" in Doric is Silent.