Now this is interesting... The Magenn company will soon be selling a highly unusual floating wind turbine. Its kind of a fusion between a watermill and a blimp.
The turbine is a specially shaped cylinder that can be filled with helium, and will rotate with as little as a 2mph breeze. It operates at much slower wind speeds than typical wind turbines... and even if it didn't, since its a balloon you can send it up a few hundred feet to get higher wind speeds.
I really like this idea, because you can place these blimps closer to the city than typical wind turbines. Also, they appear to be easy to set up, so its a great replacement for diesel generators in remote locations, or after a natural disaster.
Of course, there's the little problem with airplanes getting too close to the tether and causing a accident... that would be about as fun as cutting a high-tension power line with a pair of metal scissors. I can imagine the FAA and other government agencies being a bit concerned with levitating power plants. Magenn addresses a few of these on their safety page.
They should begin selling 4 kilowatt units this year. Those should be able to supply three average homes with electricity. To make it viable for full-scale wind farms, they need to have much larger units in the megawatt range. These are on the roadmap, but won't be available until 2011.