For NF it's still a foul to leave on the wrong side of the field for substitutes and replaced players, but not players.
Dr. Moore: We did have some interesting discussion involving a player who left the field and re-entered during the down a while ago. If you split hairs one may argue there is no foul for a player to re-enter during a down so long as he didn't violate shift or enforcement restrictions. But, I think we all agreed that would be wrong. In that thread I believe Bob M had a pretty logical case of what you should call (Illegal Sub). It had to do with a player filling his own vacancy.
http://www.officialforum.com/showthr...3&pagenumber=1