The monster's heads sometimes appear as white rectangles. I'm running version 1.21 on my G4 with ATI Radeon 9200.
We are aware of this problem and working to fix it.
Same problem with PowerBook 15" G4 1.25Ghz, Radeon 9600 and 768MB ram.
I have almost exactly the same system (only other RAM) and no white rectangles.
Strangely, I do not have this problem with the demo version. Will there be a patch sometime soon?