"The Interactive Connectivity Establishment (ICE) draft, developed by the IETF's MMUSIC working group, provides a framework to unify the various NAT traversal techniques. This enables SIP-based VoIP clients to successful traverse the variety of firewalls that may exist between a remote user and a network."
The above is from a good intro article that appeared in Network World magazine online.
The ICE draft is here.
The ICE wiki page from VoIP-info.org