So far I have never forgotten to call or email a candidate when I personally told them I would do it. I put it in my todo list and then it gets done. This "being busy" is just a lame excuse.
I guess it depends on how many candidates you interview in a day. I suspect that there is a point where taking the time to be the bearer of bad news just doesn't seem to be worth it anymore. But I guess at that point one should stop promising to call or email candidates.
If you are being asked to interview candidates (especially if you are being asked to coordinate/lead the interviews), then you should also have the time/bandwidth to do the follow-up work with the candidates, even if that means sending them a canned response.
Another comment noted that we're talking about common human decency here--a candidate took time out of their schedule to spend a good chunk of a day (likely more, counting phone screens and travel), and someone can't take a few minutes to even respond to an email. I'm not sure I'd want to work with/under someone like that.
