When the createObjectURL() method is called with a MediaStream argument, the user agent must return a unique Blob URL for the given MediaStream. For audio and video streams, the data exposed on that stream must be in a format supported by the user agent for use in audio and video elements. http://www.w3.org/TR/mediacapture-streams/#url
createObjectURL has been removed from the latest editor's draft: http://dev.w3.org/2011/webrtc/editor/getusermedia.html#change-log
Looks like URL.createObject() will be replaced by "direct assigment" mechanism. http://www.w3.org/TR/mediacapture-streams/#direct-assignment-to-media-elements