Can a mobile phone bring about contempt of court?:
Is it possible, sure.  Your question is difficult to answer because it really depends on the practice of a particular courthouse or judge.  Many federal courts require individuals with cell phones, pagers, blackberrys, etc. to leave them with the Marshal Service.

If a judge has a standing policy against such things in his courtroom, whether on or off, you could be held in contempt.  My guess is that most judges wouldn't go as far as to hold someone in contempt if it was off.  But I've personally seen  a person's phone get confiscated b/c there was a concern that the individual was attempting to take pictures of witnesses with it in a drug case (i.e.--witness intimidation).