You could power a turbine if the aircraft was already at altitude by letting gravity do the work, but you would never reclaim the amount of energy you put into the aircraft to get it to that altitude. Plus you have to then carry the mass of the generator you are using. Even if you had an airplane at speed in level flight, the drag coefficient on the wind turbine would decrease the amount of force you could apply on your propulsion system. Plus the propulsion system turning through the air would also experiance drag. Even if your conversion mechanisms were 100% efficient you still would experiance a net loss because you wouldn't be imparting additional energy intot he system
Think of it this way. if you start a non powered airplane off at 10,000 feet, put it into a perfect parabolic dive, the airplane will only be able to climb to 9,000 or so feet because of drag on the wing and fusalage. (ignoring thermals and sailplanes for the moment)
There are several proposals out there for an electric or electric hybrid passenger aircraft. Composite fusalage, electric propellers, solar cells over the entire fusalage and wing area. It wouldn't be as fast as a jet, but it theoretically would be much cheaper. They already have some long duration UAV prototypes in the works along those lines, but the main limitation at this point is our reliivly inefficient solar cells. Give things another 5 years, and if solar cell efficiency goes up significantly (which it is looking like it might, with things like the recently discovered black silicon, etc) then I would expect to see design work start on a hybrid solar aircraft. Probably somethign in the short haul class to start.
I personally would like to see a return of the semi rigid airships such as those that the new zepplin company is making, only larger, for domestic cargo delivery. You could coat the top of one of them with solar cell cloth, and if you didn't mind a slower transit you wouldn't have any fuel costs. since a large blimp would have more than enough surface area to power engines.