as the cuda use-flag pulls in dev-util/nvidia-cuda-toolkit. I guess you should mask this flag in your profiles and rekeyword k3d. of course, if dev-util/nvidia-cuda-toolkit works on ppc, you can keyword that... thanks in advance.
cuda can only be used by amd64/x86 so I've masked the USE flag for k3d in base and unmasked it in x86 and amd64 profiles (like sci-misc/boinc does). So the re-keywording has been done :)