A description of how the thing works is not convincing: You hold the iPhone into the wind, and the windmeter app determines the wind speed by the noise picked up by the microphone. How the heck can that be accurate?
I'm here to tell you it's pretty darn good. I gambled $ .99 and bought the iPhone Wind Meter App a month ago, and have tested it a half dozen times. I've compared it side by side with conventional windmeters in 20-30mph wind, I've compared it against sailing results on the water, and the thing has been both consistent and accurate. The only shortcomings I've found are that you have to hold the thing to your side and then glance at the display (because the microphone is on the front of the thing...look at me using it in the photo) and it probably shouldn't be used in rain or near splashing waves. So if you sail and have an iPhone, it's a bit of a no-brainer. Wind speed can be displayed in MPH, knots, kmph, Beaufort, meters per second, and feet per second. Note that while the "dial" portion tops out at "25+" the digital readout does not. I've seen it display 35mph. The Wind Meter App currently works only for the 3G model of the iPhone...its creators tell us they're working on 3GS and 2G versions and hope to have them available soon.
(One situation in which it consistently fails: holding it out of a car window and comparing the measured wind speed against the vehicle speed...it always reads low. I suspect this has to do with the background noise created by the tires.)
iPhones (and presumably other smart phones) are great digital "swiss army knives" for windsurfers; my own iPhone gives me the wind meter, tide charts, surf reports, forecasts from NOAA and iWindsurf, GPS-enabled maps to help me drive to new launches, access to the the local windsurfing forum, and of course a phone to call my sailing friends. If the thing was only waterproof then I'd also have a GPS tracker and speedometer, a camera (still and video) and a map to help me get back to shore in the fog. That would be really cool.
(The editor with both his iPhone windmeter and a spiffy US Windsurfing sweatshirt, at North Sea Road. Photo by Jeff Schultz.)