@siafu: This is a little trickier than what you first may think. You don't have to forward any TCP traffic, WOL goes only over UDP (if you can leave it blank, do so). You will also not be able to do any forwarding of traffic to a local computer in the normal manor, it's because the sleeping machine doesn't have an IP (yet). Only mac address'es are typically used for WOL magic packet. What you should do is to forward the packet to your subnet's broadcast address (whatever that is, depends on you subnet's class). For a class C network for instance, the broadcast address is 192.168.1.255. Now, what you should do is:
1. Set up this application to direct the magic packet to your WAN IP, fill in the mac address of your local machine to wake up (that MAC will be the payload of the magic packet, repeated 16 times).
2. Set up your router to forward UDP traffic on port 9 to the broadcast address in your network (which you will have to find out what it is for you subnet).

It does work, if you do it right, but remember you can only wake up one specific machine in your subnet this way, that's it.

I am also working on an WOL application for the N900 (in Qt) since some time ago, but it has then grown into something more than that. It is already functional like this app here, but it is also supposed to be able to set machines in suspend state, hibernate state, reboot them, shut them down and log out the user. This of course requires a daemon/service on the Linux/Windows side, but that's no problem and it already works. I just have a little more work on the PC side with a gui to the daemon/service and also a some security issues, since that becomes important as well.
Here's a screenshot for you I had lying around, in Swedish sorry to say: