How much does armor diminish damage taken? Also, how is it's durability calculated? Is it worth making a set of diamond armor or is leather just as good (seeing as leather isn't used for anything else)?
First off: http://www.minecraftwiki.net/wiki/Armor
In a nutshell, you have armor points just like you have hearts.
So you can have a grand total of 10 armor.
Armor reduces damage taken by up to the armor protection you're wearing (modified by durability, as ChrisF describes). This means, fully armored, you should be able to fall 20 blocks farther before taking fatal falling damage, and any mob hit (while at full armor) won't hurt your hearts at all.
Armour blocks damage and blocks the same amount of damage regardless of what it's made of. What does change is how long the armour will hold up:
The Crafting page has a section on armour durability. Basically each time a piece of armour protects against minimal damage it counts as 1 use, and you can get more uses out of the harder materials. The page has a table which gives the number of uses you can get out of each material.