Since you are God, why don't you just say Be.... ?
In the DWG translator (in PM) you set what the DWG unit should correspond to when uyou save the DWG file. You'll have to tell the Autocad user to set his DWGunit to the same when opening the file. DWG doesn't store "real" measurements, to my knowledge.
Oh and I just now remembered - in recent Autocads there is also a parameter called INSUNITS which has to be correct. Archicad9 and PM9 doesn't handle this correctly - it's a confirmed, old, but never fixed bug. You cannot set it in AC/PM. Your Autocad partner will have to check that one too.
I think most AutoCAD users know that once they open any file and the measurements are in decimal inches, they just go to Format/Unit and change it from Decimal to Architectural.
in the past however I used to go into each DWG file after saving it to do this change to be on the safe side.
I do view this as an AutoCAD problem and AC doesn't have to accommodate all of it's limitations.
YA.H.VE wrote: For a few drawings this could be fine, but for more than 30 its a real pain! What about AC 10? Is that steel happened? Or as you said, is something in Autocad not Archicad issue?
I think I've heard that GS has fixed the INSUNITS bug in AC10, but I really haven't tested it enough to be sure. Nonetheless, I haven't heard about any issues with my AC10-exported dwgs yet, but that may depend on the recieving end...
You should take care though, if you depend on some special pen-to-color mapping - there is supposed to be a problem in AC10/817 with that. The PenIndex translation is gone in AC10, instead you have to create a translation table (there is one delivered that delivers this function). I haven't had any problems yet, though, but that may be because I use (out of pure laziness) the standard Autocad pen set myself.