Around here (Canada) PPCs and 060s are as rare as hen's teeth, so I'm always amazed when you Europeans say you have 2 or 3 of them.
I just have an 040 (which has served me very well).
I was thinking, there are probably only about 2500 working PPC boards still around in the world, and less in use. This is just a guess, of course.
So that really caps the number of OS4.0 copies that will be sold - unless a new board is released.