It seems the 510 modem has standard NAT bindings for some protocols. The SIP protocol is bound to port 5060. Because of this, iChatAV cannot connect with audio/video, and you get either a timeout or connection failure. Examining the "connection doctor" log reveals it uses a private network IP number to connect. Obviously, this will not work.
The solution is simply to telnet to the ADSL modem and remove this binding:
telnet 10.0.0.138
Trying 10.0.0.138...
Connected to speedtouch
Escape character is '^]'.
Username : root
Password : ********
----------...
... welcome message trimmed
...
Unbind the SIP protocol and reboot the modem:
=>nat unbind application=SIP port=5060
=>config save
=>system reboot
To check whether the SIP protocol is bound in your modem use this command to show the list of bindings (only showing SIP here):
=>nat bindlist
Application Proto Port
SIP udp 5060
Now ichatAV should have no trouble connecting with audio and video!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041021083128427