I think if I was in a serious and committed relationship with someone, and camming came up as an issue, I would do what I always prescribe to do, just talk to them. Some people might have issues with it, and others wouldnt, and honestly I feel in order for it to be considered cheating the other person needs to a.) Be kept in the dark and b.) not like, agree or approve of the activity. Thus as a result of this, I can't really say if it is or isnt cheating, because it depends on the person you're in the relationship with.
However, I do (like Ara) feel that this affords pretty simple global guidelines. If you don't want your partner to find out, its deceitful, and therefore, probably cheating.