A porch ceiling isn't exactly exposed to the elements, but it must withstand more environmental adversity than an indoor ceiling. Drywall is almost never a good choice, nor is unfinished hardboard or any other highly absorbent material. That doesn't mean you can't use wood; in fact, wood is a recommended option, as long as it has a protective coating. You may also find composite, vinyl or PVC bead-board or slats that go well with your home's exterior decor.
You might chance to look up when standing on the porch of an older clapboard or brick house, and if you do, you'll probably notice a tongue-and-groove -- or bead-board -- ceiling. The boards are usually 2 or 3 inches wide, and typically milled from pine or fir. Depending on the age of the house, they may be protected with several coats of paint. This traditional ceiling covering is as popular in contemporary homes as it is in traditional houses.
Tongue-and-groove boards are appropriate for newer porches as well as older ones, but you don't have to painstakingly nail each board to the rafters as the home builders of yore did. Simply install plywood panels that are stamped to look like T&G boards. Once you give the ceiling a coat of paint, no one will be able to tell that the boards aren't real.
A Board-and-Batten Approach
You can use plywood on a porch ceiling, but in such a way that no one will notice it. Go for a board-and-batten approach by covering the seams with 1-by-3-inch fir, cedar or redwood trim. Paint the ceiling a single color or try a two-tone approach by painting the trim a complementary color.
Durable fiber cement board and hardboard options are available as siding options, and some of these work well on the ceiling of an exposed porch that must withstand humidity. Composite materials are made to be mold-resistant; any mold that happens to grow in the corners can't penetrate and is easy to wash off. Some composite panels are designed especially for ceilings and soffits -- they are smooth and not featured -- and have a hardboard core with a surface that is well sealed against moisture.
Plastic Bead Board
It isn't wood, and it isn't a composite, but vinyl or PVC bead board looks enough like wood to fool all but the most discerning eye. Choose a color that goes well with the decor, though, because these materials aren't always paintable. Plastic bead board doesn't have much structural strength, and although the boards reinforce each other when fitted together, they can sag if insufficiently nailed. To avoid this, builders often install a layer of 3.8-inch plywood first, then nail the bead board to the plywood.