This should do it:
qemu-system-ppc -cdrom /dev/cdrom -m 512 -boot d
or maybe this:
qemu-system-ppc -cdrom ~/Desktop/macos10.4.iso -m 512 -boot d
or maybe this one:
qemu-system-ppc -m 512 -M mac99 -cdrom <some file> -hdd <some file> -boot d
Please be sure to use QEMU 1.6.0 or higher when trying. Mac OS X support
was only added recently. Also stick with -hdd if you want to use a file
as the hard drive. The other hd letters don't allow Mac OS X to boot.
The only version of Mac OS X I have installed successfully was Mac OS
10.2. Maybe you might have better luck.