Since there has been a fair amount of debate on the subject, I figured why not post the actual rules:
taken from: http://usacycling.org/forms/USAC_rulebook-6.pdf
6B6. The responsibility of keeping on the prescribed course rests with the rider. A rider may not leave the prescribed course unless ordered to do so by public authorities or a race official [disqualification]
6B7. Short-cutting and/or cutting trail switchbacks or course is not permitted and may result in disqualification. A rider is required to stay on the designated trail that is marked and or specified as the race course. It is the rider's responsibility to know the designated race course. Lack of tape or barriers on the course, or misplaced signs will not constitute an excuse for cutting the course.
6F1.(f) If a rider leaves the course, the rider must re-enter the course at the same spot where the rider exited the course. If a rider is unable to re-enter at the point of exit, and gains no advantage, the Chief Referee will determine if the rider is assessed a penalty.
taken from: http://usacycling.org/forms/USAC_rulebook-6.pdf
6B6. The responsibility of keeping on the prescribed course rests with the rider. A rider may not leave the prescribed course unless ordered to do so by public authorities or a race official [disqualification]
6B7. Short-cutting and/or cutting trail switchbacks or course is not permitted and may result in disqualification. A rider is required to stay on the designated trail that is marked and or specified as the race course. It is the rider's responsibility to know the designated race course. Lack of tape or barriers on the course, or misplaced signs will not constitute an excuse for cutting the course.
6F1.(f) If a rider leaves the course, the rider must re-enter the course at the same spot where the rider exited the course. If a rider is unable to re-enter at the point of exit, and gains no advantage, the Chief Referee will determine if the rider is assessed a penalty.