CAT, A CARTOON BOOK by B. Kliban, now has 320,000 copies in print in its trade paperback edition. Two years ago, it was the stocking-stuffer Christmas present, and some of its captions (most notably the definition of the cat as "One Hell of a nice animal frequently mistaken for a meatloaf") have entered the language of humor forever. Cat's publisher, Workman, decided to offer a bookstore poster as a promotional aid, and therein lies the tail. The posters kept getting swiped out of the bookstores by cat-lovers and Cat -lovers. Other, more upright citizens wrote to Workman, asking for the posters. It was too good to pass up. A price of $1.95 was printed on it, and another drawing selected for a second poster. They sold well. Workman decided to issue a portfolio of them, six posters in a gift envelope for $5.95. Fifty thousand sets are out there now, and more are promised. In order to promote the poster set, Workman had made up a four-foot-high die cut poster of "Momcat," a marsupial tabby with a phlegmatic kitten in her pouch."Momcat" appeared everywhere and she was swiped. Now Workman is planning to offer "Momcat" for $3.95, but they're not certain what promotional aid they should offer with it . . .