What other people aren't saying is that its two hits with your hand, but you can get it down to one hit if you use a stone sword. Since they practically cost nothing, I have no problem with making a lot of them, and then they always do one hit kills from that height.
The Meaning of Life, the Universe, and Everything.
How falling damage is calculated: (# of blocks you fell) - 3 is the amount of half-hearts of damage you take.
Most mobs have 20 hp or less, so 23 is good enough to kill nearly any falling mob besides chickens and blazes and golems and enderman or enderdragons or withers.
Why all the speculation? There is this wonderful website called minecraftwiki.net, with wonderful pages that give you wonderful information.
Let me tell you how it works: You go to the website www.minecraftwiki.net and write what you need information on into the search bar in the upper right corner as briefly as possible. If I wanted to know how falling damage is calculated I would just write "Falling damage" into the search bar and that would lead me to this page. Now that I know how falling damage is calculated I would type "Health" into the search bar, that would lead me to this page. I would scroll down to get a table with all the mobs and how much health they have. Next step would be to use my brain and see how many blocks a certain type of mob has to drop to get down to how many hearts I want it to have.
Now for real, minecraftwiki.net should have a link in the launcher, there is so much information on there one couldn't even keep it in mind.
P.S: This thread is so old I don't think that anyone who has posted in this thread in 2012 will ever be looking at it again.