Gmail's voice and video chat works fine and has a pretty simple UI. I'm not sure it follows those specs completely but there are open-source clients that can connect to it. The Google client itself has pretty good Windows/OSX/Linux support as a browser plugin and with the Google Voice integration in Gmail you get a pretty full-featured UI to call both people that are on Google Talk and normal phones. Unfortunately the Google Voice stuff is still US only.
I did mention there were open-source implementations that could interface with it. I tested it against empathy (telepathy based) today and was pleasantly surprised with how well it worked. Previous attempts weren't as successful. I tried with both clients inside the same NAT though so I don't know if the firewall punching bits are as good.
Thanks for the clarification on the standards, it seems to have evolved since the early days.