If you happen to be using the Linksys Wireless G router (model WRT54G), and are having problems with Xbox live (specifically, not being able to join certain parties in Halo 2), do this:
Go to your router's setup page. (Probably http://192.168.0.1, but consult your manual if you're not sure.)
Under Administration, Management, disable UPnP.
Hopefully, you should be able to join parties now. Randy and I had been banging our heads over this issues for at least a week now. Following some random link on the Bungie website, I finally found that solution.
All I can say is, motherfucking hallelujah!
And apparently, this is a known issue, and Microsoft and Linksys are working hard on a fix. Which I'm guessing will require some kind of firmware upgrade for the Xbox, so it might never happen.