Current attack rate
Delay * --------------------- = Effective Delay
         Hasted attack rate

Your current attack rate is 100%. This is the standard speed at which you attack. Your hasted attack rate is your standard 100% attack rate, plus whatever rate increase you have from your haste item/song/spell. Dividing the current attack rate by the hasted attack rate gives you the factor by which your delay decreases. For example, 30% haste gives a factor of .769. With 30% haste, your delay would effectively be .769 of the weapon's stated delay. If your weapon had a stated delay of 24, a 30% haste item would give your weapon an effective delay of 24 * .769, or 18.46.

Note that this calculator represents the mathematical side of haste calculations. It does not take into effect any level or hard caps on haste percentages or final weapon delays.

http://cloud.prohosting.com/~talone/HasteData.htm is a great page with tons of haste info. Check it out.