present participle. "Ass cheeks on a platter" would be another way of saying it.
Related terms: back-cleavage-revealing, posterior-cleavage-revealing, rear-end-cleavage-revealing, gluteal-cleavage-revealing, non-cleavage-revealing, glimpse-of-cleavage-revealing, deep-cleavage-revealing, cavernous-cleavage-revealing, man-cleavage-revealing.
Real citation: "Schaffer is doing a beachwear collection inspired by Vegas show girls with peacock colorings and the aforementioned sequins. I love creating swimwear, she says of her low-cut V-front, butt-cleavage-revealing one-pieces."
("Behind the Seams," http://citypaper.net/articles/2003-05-15/naked.shtml)
Made-up citation: "'Plumber's-crack-defying' is a fine synonym for 'butt-cleavage-revealing,' as Sylvia Plath, John Donne, and other fine poets have demonstrated."