Combine dried pineapple and boiling water in a small bowl; let stand 15 minutes; drain thoroughly and blot dry.
If pineapple pieces are large, cut them into 1/2-inch pieces.
Heat oven to 325°.
Sift together flour, baking powder and salt; set aside.
Beat butter and sugar with an electric mixer on high speed until light, 3 minutes; add eggs one at a time, mixing well after each addition; reduce mixer to low speed; add milk, rum, and vanilla; add flour mixture, mix just until combined; fold in pineapple and nuts with a spatula.
Grease and flour an 8 1/2" x 4 1'2" loaf pan; transfer batter to pan; bake until a wooden pick inserted in the center comes out clean, about 1 1/2 hours.
Cool in the pan 10 minutes; loosen cake from the sides of the pan with a flexible knife; invert onto a wire rack and cool completely before slicing.