Late-night TV almost had its own Lindbergh baby. Authorities arrested a Montana man last week for allegedly plotting to abduct DAVID LETTERMAN'S 16-month-old son HARRY and Harry's nanny for $5 million ransom. KELLY FRANK, right, who worked as a painter on the Late Show host's isolated Rocky Mountain ranch, allegedly told an acquaintance that he had a key to Letterman's house and knew where the baby slept. That acquaintance alerted police, who charged Frank with felony solicitation. Letterman and his girlfriend Regina Lasko issued a statement calling themselves "forever grateful" to the FBI and local authorities for their efforts. It's a good thing they got the suspect too, 'cause we'd hate to see him after the posse from the Hello Deli got involved.