Attention, teens under 17: You don't have to sneak into R-rated films anymore! At select theaters in Illinois and Michigan, underage kids with parental permission can use an R-Card to see movies like ''Troy'' or last March's ''Dawn of the Dead'' without an adult. ''Parents can give their teens the freedom to watch an R-rated film if they feel they're at the appropriate maturity level,'' says Hiede Cravens, marketing manager at GKC Theatres, which introduced the card last October. (Already nearly 500 teens have signed up.) But MPAA prez Jack Valenti questions the any-R-movie-is-fine stance. ''There must be an accompanying parent,'' says the ratings czar. ''They can't do it by card because that's a blanket endorsement of every R film -- and that, of course, is wrong.''