UPDATED 19 April 2009... see the Google links at the bottom of this post.
Some quick formulas for estimating Hurricanes:
From 1013mb to 1000mb-A 1mb drop, in pressure, equals a 5mph increase in windspeed.
From 1000mb to 890mb- A 1mb drop, in pressure, equals a 1 mph increase in windspeed.
Delta P-or change in pressure: After a storm reaches 75mph. A rough estimation of the Maximum Possible Windspeed (at Flight Level or 10000 feet) may be assumed by using the following formula. 1000 - present pressure in mb= deltaP deltaP + 75 = Rough estimate of the Max Poss Wind.