Actually, the butter-side-down phenomenon has everything to do with the aerodynamics of the type of bread product used (Wonder Bread, for instance, tends to flip over more easily than does a Lender's bagel). A cat, on the other hand, has virtually the same probability of landing on its feet regardless of age, size or breed. Buttering the cat would not affect its ability to land feet-first, though it would increase the ease with which it would first wriggle out of one's arms.

The cat, therefore, would land on its feet, give itself a good washing to get the butter out of its fur, and cough up a hairball into your shoe.