I have used phone interviews as a pre-interview for many jobs I was the hiring manager for. I think phone interviews are fairly effective, but they are clearly not as effective as a video conference interview.. Webcams are pretty cheap these days, and if you are interviewing for a professional job, I think it's reasonable to expect candidates to meet the requirement of video conferencing as a substitute for the phone interview. I don't know of anyone that would hire someone without a face-to-face interview, so I don't think that next step of the interview process is going to be replaced by video conferencing anytime in the near future. 
  
If I wanted to interview people over video conferencing, I would need to make sure that the interviewees knew what tool was going to be used for the interview whether it be Skype, GoToMeeting, or something else like MSN Messenger. The interviewer would have to decide whether or not it is reasonable to expect the interviewee to know how to use the tool in advance based upon the type of job it is. I'm not sure it's feasible to expect a special education teacher to know how to use video conferencing. 
  
If I was being interviewed via video conferencing, I would prepare by doing a practice session with a friend of mine to ensure that video and sound quality was good. I would also have a phone number available as a backup to use for audio for the conference. Even on good internet connections a video conferencing tool can have some audio problems. I would make sure that my background was appropriate for the interview and that I dressed the same as if I was going to a face-to-face interview.