Beta software at present!!!
Great News for free-as-in-freedom software fans!
There is now a gNS developer's snapshot iso available for PPC from Karl Goetz:
The link to the iso is dead.
If you aren't familiar with FSF guidelines, check out the distro and the Free Software Foundation. Just know that your non-free Broadcom-driven airport or airport-extreme wireless card is not supported. You'll have to find a suitable replacement. (like a Netgear WG111v2 usb stick or similar. Trying to get a non-free Broadcom card working in gNS would defeat the whole purpose of what you are doing anyway.)