There is nothing wrong with giving an NP-complete problem as a programming challenge during an interview. I only see something wrong with expecting to find a polynomial-time solution to the problem during the interview.
An interviewer should want to see how a candidate deals with a variety of situations -- including situations that the candidate can't find an easy solution to. "Impossible" questions show how the candidate reacts when there's no simple solution. Does the candidate just give up? How many different attempts does the candidate search? How far-reaching are the solutions tried? When does the candidate ask for help -- and how? Does the candidate complain that the problem "isn't fair"?
In short, such an interview question isn't about solving P=NP... it's a psychological answer.
