When General Hospital's Tony Geary and Genie Francis appeared as Luke and Laura on Roseanne in early January, Roseanne and Tom Arnold agreed to return the favor and check into GH for a guest spot. But since then, the mercurial Arnolds have missed two tape dates. Roseanne joked that she canceled because she was in a real hospital: ''a nut barn,'' in her words. But the Arnolds' publicist said it was because of reshoots for Tom's movie True Lies. Now it's anyone's guess when, or if, the Arnolds will drop anchor in Port Charles, though a GH spokesperson is ''still very hopeful.'' The Arnolds may shake things up by playing new characters, who, GH says, ''will be intrinsic to the story line over several episodes.''