Some aerobatic model designs include a small (1-2 degrees) of dihedral in the wing to reduce adverse roll coupling with the rudder. A plane with a truly flat wing that has the rudder above the centerline will have a tendency to roll opposite the turn due to the induced moment from the rudder. Adding a small dihedral counteracts this tendency.

Dihedral also increases roll stability by increasing the lift on the "down" wing, which tends to right the plane. Anhedral has exactly the opposite effect, which reduces roll stability. An interesting bit of dihedral trivia is the B-52. If you look closely at the plane, you'll see the wing is anhedral when sitting on the ground. When the plane is loaded, the weight of the plane in flight causes the wing to become level or slightly di-hedral. A bad side effect of this design is that if the plane rolls to far, the wing looses the dihedral, and the ailerons are no longer effective enough to counter the roll. A few of them have been lost due to pilots overbanking at low altitude. If you've ever seen the video of the model B-52 that crashed in England a few years ago, it probably crashed due to the same effect.

Brad