So, a friend's twisted my arm into some HL2 Deathmatch.
Tried it this evening, and neither of us could see each other's servers running.

Anyone got any hints on how I can set a server running on my end that he can see over the wild and wooly internetz?
I've been Googling, and I'm reading lots about port forwarding.

My home set up is a 2wire 1000HG modem router connected via ethernet to an Apple Airport (the 802.11n version) that serves up the wifi for the house, which is how I connect to the world.

Any advice?

iirc correctly on 2wire you need to use a feature called 'pinholes' which is their equivalent to port forwarding. It's been over a year since my one and only experience of forwarding from their routers, and I can't remember much detail, sorry.

So you start by finding out what IP address your computer has start > run > ipconfig
Then find out what internet address you have. Just go to ipchicken.com

Set the pinholes thingy to forward the following TCP and UDP ports to your internal ip (be specific)
27000-27015 UDP and 27020-27050 TCP

Then give your friend the ip address you got from ipchicken

If he can't connect first make sure your internal address is not protected by what I seem to recall was quite an annoying little firewall on the router. Tell it to permit those ports and you should be fine.

You can back up your router's current config before you start, and then when you are done just restore that to undo any changes you have made.

If you love the experience and want to do it again every day, you can set up a dynamic dns service which will replace the minor effort of looking up your internet ip address each time you want to set up a session.