The “Today”show isn’t letting Katie Couric‘s return to morning TV go unchallenged. In the ultimate of TV teases, the network began running promos today about a secret guest who will appear Monday — “for one show only” — the same day Couric begins a week of fill-in co-hosting on cross-town and cross-country rival “Good Morning America.” And as teases go, the promo gave no hint about who it might be. Gene Shalit? Bryant Gumbel? Guess we’ll just have to wait and see.