Using the IAXclient library (http://iaxclient.sourceforge.net), VoiceAndVideo support could easily be added to Adium.
The IAX protocol is a 3rd generation VoIP protocol that uses only a single UDP port, separating signaling and payload by tagging information elements instead of using different ports. 2nd generation protocols such as H.323 and SIP spread signaling and payload over different ports which causes various problems for NAT and firewall traversal and makes configuration for end users very difficult. Using IAX means true plug and play and the connections are more robust. IAX has recently been sumitted to the IETF for approval as an official internet standard.
There is also a standard for IAX services and how they are advertised using Zeroconf (aka Bonjour, aka Rendezvous). Following this, Adium users would be able to find IAX services advertised via Zeroconf, eg. IAX phones or Asterisk servers.
