A University of Texas professor creates tiny windmills to tilt at providing electricity: The prof has developed a system with his group that uses piezoelectric crystals which, when flexed by the small pressures provided by a 10-centimeter windmill running as slow as 17 kilometers per hour, can produce 7.5 milliwatts of electricity. This could be enough to power wireless sensors.
After reading an excellent article on flywheels in Wired back in 2000, I envision a nifty future in which remote wireless transceivers could combine a small windmill, solar cells, and flywheels (instead of batteries) for a long-lived and low-maintenance power trio.
Wi-Fi, of course, drains much more than 7.5 mW, and other technologies would be even higher. But this is an interesting start. If it's cost effective someday to build tiny windmills, it may also be reasonable to build small, but not absolutely tiny ones that meet the downward spiral of wireless network power requirements in the middle.
His paper from last year can be downloaded.