Discussion on the mailing list in early June 2013 led to this proposal for a change, which seems to be acceptable:

Current:


dictionary RTCIceServer {
    DOMString  url;
    DOMString? credential;
};


Proposed:


dictionary RTCIceServer {
    sequence<DOMString> urls;  // for backwards compatibility, implementations
                        // should also handle |url|
    DOMString?  username;  // previously agreed upon
    DOMString? credential;
};

4.2.2.1 Dictionary RTCIceServer Members


username of type DOMString, nullable

If the url element of the internal array is a TURN URI, then this is the username to use with that TURN server.

credential of type DOMString, nullable

    If the url element of the internal array is a TURN URI, then this is the password to use with that TURN server.
urls of type DOMString[]

    A list of STUN or TURN URIs as defined in [STUN-URI] and [TURN-URI]. If multiple URIs are present, they represent alternate ways to contact the STUN/TURN server, listed in order of preference.

Link to original proposal: http://lists.w3.org/Archives/Public/public-webrtc/2013Jun/0075.html