As Luis has observed, there is now, thanks to Andreas Røsdal, support for zeroconf game server discovery in gnome-games. Specifically both iagno and gnibbles can take advantage of it. Unfortunately, up until twenty minutes ago, the build was broken because I had forgotten to add a file to CVS. It should work now. I hope.
There are a few warnings though: The networking code seems to work fine, but the UI still needs some work. If you know what you are doing, it is fine, but it doesn’t really live up to the GNOME 2 “Just Works” philosophy. You probably want to adjust the preferences the first time you run a network game since the defaults don’t really work (unless you have set up a game server on localhost, run game-server.py to do this). The zeroconf option works a lot better and will eventually become the default. Someone has to set up a local game and the other person has to choose from the list of local games. The fact that I have to explain all this is what is wrong with the UI :).
Also, it hasn’t been tested for security, so don’t start a network game on an untrusted network. Always make sure that you can physically lay your hands on anyone who may decide to play silly buggers.
If you don’t have libhowl then the zeroconf stuff won’t be built. If the local game options are greyed out, then the build process didn’t find libhowl.