>ofb0 at pci0 dev 17 function 0: vendor 0x1002 product 0x5654
>ofb0: 832 x 624, 8bpp
>
>And later fails in atiprobe.c (but without a proper error message). This
>would suggest to me, that the card type is not supported, but the XFree 
>documentation says it is.

Mach64(s) are not supproted (on macppc) because they don't have VGA BIOS,
which is required by XFree86.

(Of course it is not executed directly, but there are some parameters in it.)