DOWN TO EARTH.(Joan of Arcadia )(Coupling)(Television Program Review)

It sounds like a sure miss--a show about a teen-ager who talks to God, a God who takes the form, variously, of a teen-age boy in jeans and a corduroy jacket, a school-cafeteria worker, an electrician, and a little girl playing ball. But "Joan of Arcadia," a new CBS show on Friday nights, pulls it off, without being bland or treacly or embarrassing. The only person who's occasionally embarrassed is the sixteen-year-old Joan (Amber Tamblyn), when God, to prove to her that he actually is God, tells her some secret about herself that no one else could possibly know--that, for example, the song from "Titanic" makes her cry when she's alone. In "Joan of Arcadia," God manifests ...