iPhone owners will be aware of the increasingly large and useful number of applications that can be downloaded to the phone, they're cheap too not least this wind meter function available from the iTunes Store for just 59p. 
The software uses sound conversion via the phones microphone to gauge wind speed between 0-28mph and provide an average reading. The software wil easily convert between MPH, Knots, Kilometers per hour, and Beaufort.
It sounds like a handy feature and it's now available for all iPhone types including the new 3GS phones.