Brush a 13 x 9 baking pan with some of the butter. Unroll phyllo dough; trim to fit into the pan. In a large bowl, combine the walnuts, rhubarb,sugar and cinnamon.
Layer 5 sheets of phyllo dough in prepared pan, brushing each with butter.(Keep remaining phyllo covered with plastic wrap and a damp towel to prevent it from drying out.) Sprinkle with a third of the nut mixture. Repeat layers twice. Top with remaining phyllo dough, brushing each sheet with butter.
Using a sharp knife, cut baklava into 1 1/2 in. diamond shapes. Bake at 350 degrees for 35-40 minutes or until golden brown. In a small saucepan, heat honey over low heat until thinned. Pour over warm baklava. Cool completely on a wire bakers rack. Cover and let stand for several hours or overnight.
If using frozen rhubarb, measure it while still frozen, then thaw completely. Drain in a colander, but do not press liquid out.