BUTT SERIOUSLY, FOLKS

Is CBS' top-rated The Young and the Restless going bottoms up? It sure looks that way: Y&R, with no advance notice, has become the first soap to flout the daytime taboo against men's bare derrieres. Doug Davidson, who plays mensch detective Paul, has already gone au naturel, hunky businessman Brad (Don Diamont) will follow suit in a special Oct. 26 prime-time episode, and other Y&R stars (including -- no joke -- Philip Moon) are set to turn their other cheeks too. ''What's the big deal?'' Diamont says. ''It's tastefully done.'' Besides, he sniffs, his scene -- which shows him walking into a shower-''is not about my tush.'' Diamont, one of People's 50 Most Beautiful People in 1990, says he's not worried about protecting his flanks. ''I go to the gym six days a week,'' he says, ''so my butt's in good shape. Although a cheek implant did cross my mind.''