Which OS are you using on your SheevaPlug? If it's Ubuntu or Debian you must have a /etc/network/interfaces file.
And I know close to nothing about any Apple hardware, but I'll try to look up some info on how to forward ports.
Sorry my bad, i didn't see it at first, here it is:
# Used by ifup(8) and ifdown(8). See the interfaces(5) manpage or
# /usr/share/doc/ifupdown/examples for more information.
iface lo inet loopback
iface eth0 inet dhcp
I run ubuntu. I'm going to try on another router later today, so i can find out if it is a problem with the router.