I have the same problem – kernel panic (gray screen) vith Snow Leopard and c4d R11.5. My machine is older iMac 24 (aluminium), 2,4 GHz with ATI HD2600.
Interesting is, that this problem emerged after upgrading OS X from 10.6 to 10.6.1. or 10.6.2 (not sure). Under original 10.6 it was, if I can remember, without problems.
Now I’m trying running cinema under 32 bit mode – Iĺl write here if this solves this annoying problem.